Research Article

Transportation Research Record2020, Vol. 2674(2) 146–157� National Academy of Sciences:Transportation Research Board 2020Article reuse guidelines:sagepub.com/journals-permissionsDOI: 10.1177/0361198120905842journals.sagepub.com/home/trr

Improving Railway Operational Efficiencywith Moving Blocks, Train Fleeting, andAlternative Single-Track Configurations

Adrian Diaz de Rivera1, C. Tyler Dick1, and Leonel E. Evans1

AbstractWith installation of positive train control (PTC) on many U.S. rail corridors, Class I railroads may soon leverage these invest-ments in communications network infrastructure to implement ‘‘advanced PTC’’ systems incorporating moving blocks. Traincontrol with moving blocks can benefit operating strategies that dispatch fleets of multiple trains running at minimum head-ways. On single-track corridors with passing sidings long enough to hold multiple trains, fleeting may increase the efficiencyof train meets, reduce train delay, and yield incremental capacity benefits. Alternative single-track configurations with fleet-length sidings at double the spacing of conventional single-train sidings can facilitate these operating strategies while minimiz-ing additional track infrastructure and associated capital and maintenance costs. To investigate the operational synergiesbetween moving blocks, fleeting, and longer but less frequent sidings, Rail Traffic Controller software is used to simulate andcompare the delay performance of train operations on representative rail corridors for different combinations of fleetingstrategy, train control system, siding configuration, and freight traffic composition. Operating fleets in conjunction with mov-ing blocks produces the lowest overall train delay in specific cases of low schedule flexibility and heterogeneous traffic. Withmore efficient meets, moving blocks and/or fleeting primarily benefit low priority trains that typically wait for opposing trafficduring train meets. Such incremental line capacity benefits have short-term financial consequences as they allow additionalcapital investments in double track to be deferred. Knowledge of train delay performance under moving blocks and fleetingwill aid railway practitioners evaluating investments in advanced PTC systems and track infrastructure expansion.

Positive train control (PTC) is a ‘‘system designed to pre-vent train-to-train collisions, over-speed derailments,incursions into established work zone limits, and themovement of a train through a switch left in the wrongposition’’ (1). In the U.S.A., PTC is mandated on mostrailway mainlines where passenger services operate orhazardous materials are transported, covering 54,000route-miles of track. All Class I railroads expect to fullyimplement PTC by the end of 2020 at a cost of over $10billion (2).

Many different train control technologies and archi-tectures can satisfy the legal requirements of PTC. TheNorth American rail industry is implementing anoverlay-type system that enforces movement authoritiesgranted by existing control systems through track war-rants or wayside signals and centralized traffic control.PTC systems consist of three main elements: (i) a loco-motive onboard system that monitors train location, cal-culates train-specific braking distance, and automaticallystops the train before violating movement authority orother restrictions; (ii) a wayside system that transmits

track signal, track circuit, and switch status to theonboard system; and (iii) a back office server integratedwith the network operations center that stores informa-tion such as speed restrictions or movement authoritiesand communicates with the onboard system (2). Arobust digital communications link is required to trans-mit information between the three elements. Precise trainlocation, direction, and speed information are providedby GPS signals, inertial locomotive navigation systems,and/or transponders in the track (3–5). Once fully imple-mented, PTC technology may provide the foundationfor a number of efficiency improvements railroads areactively investigating, such as one-person crews, moving

1Rail Transportation and Engineering Center (RailTEC), Department of

Civil and Environmental Engineering, University of Illinois at Urbana-

Champaign, Urbana, IL

Corresponding Author:

Adrian Diaz de Rivera, adriand2@illinois.edu

blocks, autonomous trains, or the elimination of existingwayside signals (6, 7).

Advanced PTC

Installation of a robust communications and systemsinfrastructure across the North American rail network,along with train location systems onboard locomotives,provides the opportunity for future upgrades to‘‘advanced PTC’’ control systems with virtual or movingblocks, also known as ‘‘PTC 2.0.’’ Throughout its devel-opment, PTC has been described as having both safetyand business benefits (8). However, as currently imple-mented, PTC systems are primarily safety overlays onexisting fixed block control systems with limited businessbenefits. Advanced PTC systems promise both toimprove safety and to increase line capacity on existingtrack infrastructure, primarily by increasing the avail-ability and accuracy of operational and train locationinformation, and reducing the minimum separationrequired between trains (9).

Existing signalized train control systems maintaintrain separation by dividing track segments into a seriesof fixed control blocks that can usually only be occupiedby one train at a time. Wayside signals at the entrance toeach block communicate information about block occu-pancy to the train crew with varying indications progres-sing from stop to clear. Block lengths are set for thebraking characteristics of a design train, which is typi-cally the train with the poorest braking performance thatis expected to operate regularly over the line. If a trainrequires significantly shorter braking distance than thedesign train, it must still maintain the same fixed separa-tion distance from occupied track ahead. Block occu-pancy information is only available at discrete points(i.e., where wayside signals are located). At these loca-tions, the presence of a train ahead is communicated tothe train crew, but no information is provided on howclose that train may be to clearing a signal block.Because of this uncertainty regarding when upcomingblocks will clear, the train crew must maintain moreseparation between themselves and the train ahead thanis physically necessary to ensure safety.

Adding an aspect to the primary signal progression,such as transitioning from ‘‘3-aspect’’ to ‘‘4-aspect’’ sig-nals, addresses some inefficiencies of fixed blocks. Theadditional indication provides flexibility for trains withdifferent braking characteristics, allowing a train to pro-ceed past an advance warning signal (such as advanceapproach) at maximum authorized track speed if thetrain requires less braking distance than the design train.Generally, increasing the number of aspects reducesblock size, decreases minimum train separation, and mayincrease capacity by more closely matching block length

to train length and train-specific braking characteristics.In advanced PTC systems with moving blocks, there areno fixed block boundaries, and the number of aspects istheoretically infinite since the boundaries of a train’smovement authority are customized to the train’s length,its predicted absolute braking distance, and a margin ofsafety. Using a train’s real-time location, the movementauthority boundaries constantly update to define a ‘‘mov-ing block’’ protecting the train as it travels along themainline. In simple cases, moving blocks can reduce min-imum train distance headways by as much as 50% belowa 3-aspect fixed block control system, and 33% below a4-aspect system (9). On mainline railways in Europe,implementation of European Train Control System(ETCS) Level 3, a moving block system, has been shownto increase line capacity by 35% to 40% over ETCSLevel 1, which is a safety-focused system overlaid onexisting fixed block signals (10).

Advanced PTC systems can take advantage of theseheadway benefits to increase line capacity withoutexpanding track infrastructure. Such systems may alsoallow existing wayside block signals, and their associatedmaintenance costs, to be eliminated. However, advancedPTC requires more sophisticated and reliable communi-cations and network capabilities than those being imple-mented for overlay-type PTC systems. The additionalinvestment in advanced PTC must be justified by thepotential business benefits resulting from decreased trainheadways and greater train location data availability,resolution, and accuracy.

Train Fleets

Fleets of trains are groups of two or more trains withsimilar performance characteristics moving in the samedirection at minimum headways. When trains operate infleets on single-track, opposing traffic must wait for theentire fleet to pass before proceeding. Since NorthAmerican freight rail operations do not follow a pre-planned timetable with prescribed train meets andovertakes, trains are often fleeted on an ad hoc basis bydispatchers managing congested rail corridors. Forexample, on lines with large amounts of double track, adispatcher may successively route several trains operatingin the same direction over a single-track section beforeallowing trains to proceed in the opposite direction.Another case is fleets of trains traversing a congestedurban rail network or freight terminal with numeroustraffic conflicts. Fleet operations clear train queuescaused by capacity bottlenecks in these cases. Similarly,holding freight trains outside urban areas to avoid peakperiods of commuter rail operations can create trainfleets.

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Besides operational convenience, there may be funda-mental capacity and performance benefits to operatingtrains in fleets on single-track corridors with long passingsidings. Fleeting may be able to resolve meet conflictsmore efficiently along a particular corridor. For exam-ple, consider a meet between two fleets, each composedof two trains. At a location with a long siding, the fourtrains can pass each other in a single interaction. Hadthe trains been dispatched individually, four separatetrain meets must occur. Separate meets create inefficien-cies because there is a fixed delay imposed by each meetregardless of the number of trains involved (11). For thelower-priority train, each meet requires the train tobrake, enter the siding, and clear the main track beforethe higher-priority train arrives. Once the higher-prioritytrain has passed, the lower-priority train waits for therelevant signal instruments to line a route before exitingthe siding and accelerating back to track speed. Withtwo-train fleets, each lower-priority train experiences onefewer braking and acceleration cycle than when eachopposing train is met separately. However, this reductionin acceleration and braking delay may be offset by theadditional time the lower-priority train must wait toaccount for the time separation between the two higher-priority trains being met. Since moving blocks reducetime headways within fleets, this trade-off is minimized,and there are potential synergies between advanced PTCsystems and fleet operations on corridors with single orpartial double track.

Fleets, especially when used in conjunction with vir-tual or moving blocks, can also be used to mitigate lossof capacity because of temporary track outages, speedrestrictions, or delays (12). In these cases, there will likelybe a queue of trains waiting to traverse a slow-speedsingle-track section. While fleeting is possible under fixedblock wayside signals, there is a fixed minimum trainseparation regardless of actual traffic speed and trainbraking characteristics. Trains often must maintainunnecessarily long headways since the signal spacing isdesigned for a poorly performing train operating at max-imum authorized track speed. Under moving blocks,trains operate at headways appropriate for actual operat-ing speed and braking characteristics, and fleeting ismore effective for recovering from disruptions (13).

To operate fleets in both directions for the length of asingle-track mainline corridor, passing sidings must belong enough to accommodate entire fleets. Otherwise,when two fleets meet, the lower-priority fleet must bebroken up and accrue delay waiting for the higher-priority fleet in two separate sidings (14). Once the trainsin a fleet become separated, any benefits of fleets at mini-mum headways are lost for all subsequent meets involv-ing those trains. In an effort to take advantage of theeconomies of scale offered by increasing train lengths,

many Class I railroads are actively lengthening sidingsand constructing new longer sidings on single-trackmainlines (15). In addition to accommodating longertrains, such sidings could be used to hold fleets of shortertrains for meets and passes. To improve operational flex-ibility, certain railroads have also installed double-length‘‘super sidings’’ with mid-siding crossovers that are idealfor meeting two-train fleets (16). Although there aremany practical, engineering, environmental and physicalconstraints that may prevent a particular siding frombeing extended to accommodate fleets, previous researchon operation of over-length trains that exceed the lengthof passing sidings suggests that fleets can be operatedefficiently while only extending a fraction of the passingsidings along a corridor (17, 18).

Previous Research

Dick et al. examined the relative capacity benefits ofmoving blocks in the North American freight railwaycontext by simulating and comparing operations underfixed and moving block control systems across variouscombinations of traffic volume, traffic composition, andpercent second main track (9). Operations under movingblocks exhibited consistently lower average train delaythan those under fixed block signals. Moving blocksdemonstrated the greatest benefit on lines with a highproportion of double track where headway controlscapacity. This study advances this previous work by con-sidering additional factors including train fleeting strate-gies, different single-track infrastructure configurations,and schedule flexibility. Dick also developed simple for-mulas to calculate capacity of a single-track railway linewith passing sidings for bulk unit freight trains dis-patched individually and in fleets (19). Operating fleetsunder fixed blocks with sidings long enough to hold twotrains was found to increase capacity slightly over dis-patching trains individually. Fleeting trains under mov-ing blocks resulted in almost 50% greater capacity thanfleeting trains under fixed blocks. A primary factor wasthat moving blocks effectively decrease the length ofpassing siding required to allow a running meet wheretrains do not stop.

Dingler et al. used similar simulation methods toinvestigate the root causes of train delays on a typicalNorth American freight rail corridor, finding meets to bethe leading cause of delays (20). While time stoppedaccounted for the greatest proportion of meet delays,braking and acceleration cycles also contributed signifi-cantly to delay. Higher levels of traffic heterogeneitywere associated with more complex conflicts and ineffi-cient meets. Gorman performed statistical analysis ondata from a Class I railroad to determine the factors con-tributing to congestion-related delay, identifying train

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conflicts such as meets and passes as primary contribu-tors (21).

Various researchers have examined the potential forvirtual coupling, which would allow trains to operate inplatoons at very short headways based on relative brak-ing distances, to increase capacity (22, 23). Virtual cou-pling can decrease minimum distance headways by 66%compared with a moving block system conforming toETCS Level 3. One prerequisite for virtual coupling isautomatic train operation, which is currently being devel-oped and implemented on mainline passenger and freightrailways (24, 25). Both virtual coupling and partially orfully automated trains would substantially boost theeffectiveness of fleeting trains on mainline corridors. Tocontrol the experimental factors and provide a morebasic comparison of fleeting strategies in a typical heavy-haul freight rail operating environment, however, thisstudy only considers driver-operated trains controlled byfixed block and moving block control systems.

Research Questions

This research aims to investigate the potential for syner-gies between advanced PTC, planned fleeting of trains,and track configuration to improve delay performanceand line capacity of a typical North American Class Imainline corridor. To accomplish this goal, this paperaddresses the following research questions:

� How do different train fleeting strategies underfixed and moving block control systems on alter-native single-track configurations with less fre-quent but longer fleet-length passing sidings affecttrain delay performance?

� Are the benefits of certain combinations of controlsystem, fleeting strategy, and siding configurationsensitive to traffic composition and scheduleflexibility?

Methodology

This research conducts simulation experiments to deter-mine train delay responses for various factorial combina-tions of fleeting strategy, track configuration, trafficcomposition and schedule flexibility on a representativesingle-track rail corridor with passing sidings under 4-aspect fixed block and moving block control systems.

Rail Traffic Controller

The train delay response for each experimental scenariodescribed in subsequent sections is determined using RailTraffic Controller (RTC) simulation software. RTC is arailway traffic simulation software used by most

passenger and Class I freight railways, consultants andrail capacity researchers in North America (26). RTCsimulates train movements on mainline rail corridors byemulating realistic real-time decisions made by train dis-patchers to resolve train conflicts according to NorthAmerican train control systems and operating practices.General RTC model inputs include track layout, signal-ing, train plan, train characteristics, and grade. In 2017,the developers of RTC added the capability for RTC tosimulate rail corridors operating under moving blockcontrol systems. This research takes advantage of thisnew detailed simulation capability to investigate theoperational effects of dispatching trains in fleets on dif-ferent track configurations under fixed and movingblock control systems.

Study Parameters and Experiment Design

Simulation experiments considered a 242-mi single-trackmainline with passing sidings intended to be representa-tive of a typical North American Class I freight rail cor-ridor (Table 1). While the distribution of passing sidingschanged according to the experiment design, the totalnumber of track-miles, including 50mi of siding track,remained constant at 292mi. Keeping track-miles con-stant minimizes differences in track maintenance costsbetween scenarios, along with the construction cost tobuild-out a new corridor or significantly upgrade anexisting corridor with insufficient passing sidings. Forscenarios where a fixed block control system was used,the signal block length was set at 1 mi.

Rail traffic on the freight-only study corridor consistsof 48 trains per day in total, divided between intermodaland unit coal trains (Table 2). These two train types werechosen for their ubiquity on North American railwaysand significant performance differences. Intermodaltrains are generally shorter and lighter than unit coaltrains, with better braking and acceleration characteris-tics. Trains were dispatched with an even directionalsplit. The baseline train plan includes train departuresevenly distributed throughout the day, and every traintraverses the entire corridor without planned stops.

The experiment design includes five variable factors:train control system, siding infrastructure configuration,

Table 1. Route Infrastructure Parameters

Parameter Characteristic

Route-miles 242 miTrack-miles 292 miTrack layout Single-track with sidingsNumber of curves 0Grade 0.0%Maximum authorized speed 50 mph

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fleeting strategy, traffic composition, and schedule flexi-bility. Each factor was simulated over a range of valuesor ‘‘levels’’ (Table 3) in a fractional-factorial design. Afractional-factorial design was used to reduce the scopeof the simulation experiment and eliminate combinationsof factors not of primary interest given the researchquestions.

Two train control systems enabling different mini-mum headways within fleets are examined for all combi-nations of other factors: centralized traffic control (CTC)with 4-aspect wayside block signals, and advanced PTCwith moving blocks. Four-aspect CTC represents one ofthe best performing conventional fixed block train

control systems widely adopted across the NorthAmerican rail network. Each simulation scenario usesone control system for the entire corridor. It is hypothe-sized that fleeting trains under moving blocks will per-form better than under fixed blocks or without fleeting.

This study examines the feasibility and effectivenessof three different fleeting strategies: no fleets (all trainsdispatched individually), trains dispatched as two-trainfleets in one direction, and trains dispatched as two-trainfleets in both directions. The RTC dispatching logic wasset to favor keeping fleets together and trains in fleetswere dispatched together for the relevant scenarios.However, any individual fleet could be broken updepending on the specific sequence of train conflictsencountered. Larger fleets of more than two trains couldalso be formed en route. The three fleeting strategies aresupported by two different siding infrastructure config-urations: 2-mi long sidings located every 10mi capableof holding one train, and 4-mi long sidings located every20mi capable of holding a two-train fleet.

Examining the first three experiment design factorsreveals four primary operating scenarios of interest wherethe siding infrastructure matches the fleeting strategy,resulting in substantially different train meet sequences(Figure 1). This experiment focuses on these four primaryscenarios:

� 4-aspect fixed block signals, 2-mi sidings every10mi, trains dispatched individually (4A 2 3 10No Fleets) (Figure 1a)

� Moving blocks, 2-mi sidings every 10mi, trainsdispatched individually (MB 2 3 10 No Fleets)(Figure 1b)

� 4-aspect fixed block signals, 4-mi sidings every20mi, trains dispatched in fleets (4A 4 3 20Fleets) (Figure 1c)

� Moving blocks, 4-mi sidings every 20mi, trainsdispatched in fleets (MB 4 3 20 Fleets) (Figure1d)

Different traffic compositions are included to investi-gate the effect of heterogeneity and relative train charac-teristics on the ability of fleeting, advanced PTC, andtrack configuration to improve performance. The FastFreight case consists of 75% intermodal trains, the EvenFreight case has equal numbers of intermodal and unitcoal trains, and the Heavy Freight case is 75% unit coaltrains. Cases with higher proportions of intermodaltrains are expected to perform better than cases withmore unit coal trains since the intermodal trains are ableto execute train meets more efficiently.

North American freight railways generally do notoperate to strict timetables (27, 28). Instead, trains havesome amount of schedule flexibility where a train may be

Table 2. Train Parameters and Characteristics

ParameterIntermodal

trainUnit coal

train

Locomotive power (hp) 4,380 4,380Locomotives per train 4 3Number of railcars 80 105Train length (feet) 5,250 6,520Train weight (tons) 9,680 14,280Hp per ton 1.81 0.92Average RTC priority assignment 6,000 3,000

Note: RTC = Rail Traffic Controller.

Table 3. Experiment Design Factor Levels

Factor Levels Level specification

Train controlsystem

2 4-aspect CTC with fixed blocks(4A)

Advanced PTC with movingblocks (MB)

Sidinginfrastructureconfiguration

2 2-mi sidings every 10 mi(2 3 10)

4-mi sidings every 20 mi(4 3 20)

Fleeting strategy 3 No fleetsTwo-train fleets in one direction

(EB Fleets)a

Two-train fleets in bothdirections (Fleets)

Traffic composition 3 Fast freight: 75% intermodal,25% unit

Even freight: 50% intermodal,50% unit

Heavy freight: 25% intermodal,75% unit

Schedule flexibility 3 Low: 6 10 minMedium: 6 60 minHigh: 6 360 min

Note: EB = eastbound. CTC = centralized traffic control; PTC = positive

train control.aTwo-train fleets in one direction is not included in the primary

experiment design.

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dispatched earlier or later depending on a range of fac-tors such as crew and equipment availability, networkcongestion, lading availability, and yard and terminaloperations. Since train meets and other conflicts are nottypically planned, increasing schedule flexibility has beenshown to be detrimental to mainline performance andcapacity (28, 29). To determine how this schedule uncer-tainty may influence the four primary scenarios of inter-est, three levels of schedule flexibility are included in theexperiment design: ‘‘Low’’ with +/– 10min of scheduleflexibility, ‘‘Medium’’ with +/– 60min, and ‘‘High’’ with+/– 360min of flexibility. The amount of schedule flexi-bility defines the period of time around the planned traindeparture time over which trains (or fleets) randomlydepart. For example, if a train is planned to depart at9:00 a.m. with 60min of schedule flexibility, on a givenday of a particular simulation, the train will randomlydepart between 8:00 a.m. and 10:00 a.m. according to arandom uniform distribution. All train plans are rando-mized outside RTC to ensure that, for scenarios withfleets, once the departure of the first train is randomized,the second train in the fleet departs at a 6-min headway

under fixed blocks and 4-min headway under movingblocks. No scenarios with zero minutes of flexibility (i.e.,structured schedules) were examined in an effort to over-come potential biases in the baseline schedules.

Analysis and Hypothetical Relationships

Train delay is the primary operational performancemetric output from each RTC simulation run. Followingtypical North American practice, RTC calculates traindelay as the difference between the actual running timefor a given train, and its minimum running time operat-ing at maximum authorized speeds without any trainconflicts or other impedances. The actual running timeincludes the time a train is in motion, delays departingthe origin terminal, and any delays incurred from trainmeets, passes, and other conflicts regardless whether theconflict is planned or not. Average train delay can beused to estimate capacity, with several Class I railroadsusing 60min of average train delay per 100 train-miles asan indicator of a line operating at capacity.

For each scenario in the experiment design, 30 simula-tion seeds were generated. Each seed consisted of onesingle-day schedule repeated for three days, including onewarm-up day and one cool-down day. Average train delayper 100 train-miles was recorded for each simulation. Incases where RTC determined a particular seed of a sce-nario was infeasible, the relevant seed was also deemedinvalid. Since invalid cases likely represent the worst-performing replicates of each scenario, the 10 seeds withthe highest average train delay from each scenario werediscarded, including any invalid seeds. Scenarios withmore than 10 invalid seeds were deemed invalid overall.Finally, at a p-value of 0.05, confidence intervals were cal-culated to represent the variation in average train delay forthe remaining 20 replicates.

Four hypothetical average train delay distributions forthe four primary scenarios were developed for possibleexperimental results. The control outcome is no signifi-cant improvements even with fleeting and moving blocks(Figure 2a). Based on previous research (9, 10), it isexpected that moving block implementation will providean incremental delay benefit relative to fixed blocksbecause of reduced train headways and more efficientconflict resolution (Figure 2b). Fleeting trains can alsoaffect corridor performance depending on a large numberof factors including train length, traffic heterogeneity,train braking characteristics, traffic level, and maximumauthorized track speed. There is likely a threshold factor-ial combination beyond which fleeting trains producessignificant overall benefits by reducing delay (Figure 2c).Below this threshold, particularly when combined withfixed block signals, fleeting could produce increased traindelay (Figure 2d).

Figure 1. Typical train meet sequences for operatingenvironments with 2-mi sidings every 10 mi and no fleets under(a) 4-aspect fixed blocks and (b) moving blocks; and 4-mi sidingsevery 20 mi with fleets under (c) 4-aspect fixed blocks and (d)moving blocks.

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Results

Average train delay values and 95% confidence intervalsfor each operating scenario were determined from RTCsimulation outputs and grouped by level of schedule flex-ibility. Values of train delay are considered significantlydifferent if their respective confidence intervals do notoverlap. Higher train delay is indicative of a more con-gested, poorly performing scenario.

Primary Scenario Delay Response

For the Even Freight traffic mixture, the worst-performing scenario operated under 4-aspect CTC

without fleets on 2-mi sidings spaced every 10mi (4A2 3 10 No Fleets) (Figure 3). Consistent with pastresearch (28, 29), increasing schedule flexibility producedincreases in average train delay. At low schedule flexibil-ity, the delay distribution shows benefits from both mov-ing blocks and fleets. At high flexibility, results showbenefits from moving block implementation only, largelybecause the moving block with fleets scenario degradesquickly with increasing schedule flexibility. One possibleexplanation is that since the fleets consist of two trainsdeparting together, as schedule flexibility increases, thereis a greater likelihood of a large number of trains cluster-ing at specific times to create complex train conflicts.

Figure 2. Hypothetical delay distributions for the primary operating scenarios depicting: (a) no delay benefits from moving blocks orfleets, (b) delay benefits from moving blocks only, (c) delay benefits from both moving blocks and fleets, and (d) disbenefit of fleets.

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In contrast, under moving blocks with the 2 3 10 sidingconfiguration and no fleets (MB 2 3 10 No Fleets), flex-ibility has a much less pronounced effect. The combina-tion of more frequent sidings and moving blocks is morecapable of handling traffic peaks caused by schedule flex-ibility. Generally, for the Even Freight traffic mix, thefixed block scenarios performed the same or worse thanthe moving blocks scenarios, building on previous find-ings that moving blocks allow for reduced train delay onexisting infrastructure (9).

Examining the Even Freight results by train type sug-gests that intermodal trains (Figure 4a) have a differentresponse to the various experimental scenarios than unitcoal trains (Figure 4b). Unit trains incur significant delayin the 4A 2 3 10 No Fleets scenarios while intermodaltrains generally have the lowest delay for the same sce-narios. Unit train delay appears to be more sensitive tochanges in control system and fleeting strategy thanintermodal train delay. At a low level of schedule flexibil-ity, fleeting and moving blocks benefit unit trains but donot improve the performance of intermodal trains. Tounderstand this result, consider that the leading cause ofdelay on single-track routes is time spent stopped in sid-ings for meets with higher-priority trains (20, 30). If anetwork is capacity-constrained, train meets tend to takelonger, and the priority of a train has a much greatereffect on its performance. The dispatcher will stronglyfavor higher-priority trains in conflicts while toleratinggreater delays for slower, lower-priority trains. This islikely the situation for the 4A 2 3 10 No Fleets scenarioswhich are expected to have the lowest line capacity (9).By introducing fleets and moving blocks to the network,the train meet process becomes more efficient with lessdelay incurred by the stopped train. Since there is a lowerpenalty associated with each train meet, it becomesacceptable for the dispatcher to stop a higher-priority

intermodal train for a small delay if the overall corridorefficiency is improved. Therefore, the lower-priority unittrains benefit significantly more from the introduction offleets, moving blocks, or both, than the already high-performing intermodal trains.

The overall average train delay values for the FastFreight traffic mix, dominated by intermodal trains, donot match any of the predicted delay distributions but domatch the delay pattern for intermodal trains from theEven Freight traffic mix (Figure 5a). Correspondingly,the delay values for the Heavy Freight traffic mix, domi-nated by unit coal trains, follow the same pattern as thedelay values for unit trains from the Even Freight trafficmix (Figure 5b). Overall, average train delay valuesincreased for all scenarios as the traffic composition tran-sitioned from more intermodal trains to more unit trainsacross all levels of schedule flexibility.

With the Heavy Freight mix (Figure 5b), the fleets sce-narios performed similarly regardless of control systemand were slightly better than the MB 2 3 10 No Fleetsscenario at low levels of flexibility. A possible explana-tion is that unit coal trains are likely to be similar to the

Figure 3. Average train delay per operating scenario for EvenFreight traffic composition under different levels of scheduleflexibility.

Figure 4. Average train delay per operating scenario for (a)intermodal trains and (b) unit coal trains within the Even Freighttraffic composition under different levels of schedule flexibility.

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design train used to set the lengths of signal blocks forthe fixed block control system. Since the unit train brak-ing distances more closely match the signal block lengths,the amount of excess fixed block train separation causedby a disparity between actual and design train safe brak-ing distances is reduced, decreasing the incrementalcapacity benefit of moving blocks. Since the HeavyFreight traffic mix has a higher proportion of unit coaltrains, the disparity between minimum headways underfixed blocks and moving blocks is lower.

Partial Fleeting

When trains were dispatched in two-train fleets in onedirection, but individually in the other direction, the 4A2 3 10 EB Fleets scenario has the highest overall traindelay for the Even Freight traffic mix under medium(+/– 60min) schedule flexibility (Figure 6). Examiningthe results by direction and train type reveals that botheastbound and westbound unit trains experience veryhigh delays in this scenario. The 4A 2 3 10 EB Fleetsscenario also has the highest delay values for eastboundintermodal trains but the lowest delay values for

westbound intermodal trains. Since sidings cannotaccommodate entire fleets, low priority unit train fleetshave a high chance of breaking up en route (14).Meanwhile, high priority intermodal train fleets are likelyto be routed nonstop through the corridor, imposingcompounding delays on stopped trains. In a scenariowith constrained capacity, intermodal trains are givenstrong preference in meet conflicts at the expense of veryhigh delays for both individual and fleeted unit trains,consistent with previous findings (Figures 4a and b).

Implementing moving blocks improves the effective-ness of partial fleeting, leading to significant reductionsin delay for fleeted eastbound trains and particularlylarge improvements for fleeted eastbound unit trains(Figure 6). For the same scenarios, westbound intermo-dal trains incurred significantly higher delays than in the4A 2 3 10 EB Fleets scenario. One reason is that despiteequal directional priorities, the fleeted eastbound direc-tion generally receives preference in meet conflictsbecause it is more costly to hold or break up the fleetthan to hold an individual westbound train. Looking atoverall train delay results, the MB 2 3 10 EB Fleets sce-nario, which combines the flexibility of more frequentsidings with the benefits of fleets with moving blocks,performed the best.

A key benefit of partial fleeting is the ability to utilizeexisting siding infrastructure that is only capable ofaccommodating a single train. For medium (+/– 60min)schedule flexibility, an Even Freight traffic mixture, andthe same siding configuration, the overall results can becompared with the corresponding values from the pri-mary 4A 2 3 10 No Fleets and MB 2 3 10 No Fleetsscenarios. Partial fleeting under moving blocks results in

Figure 5. Average train delay per operating scenario for (a) FastFreight and (b) Heavy Freight traffic compositions under differentlevels of schedule flexibility.

Figure 6. Average train delay per operating scenario, direction,and train type for Even Freight traffic composition with trains inone direction (eastbound) dispatched as fleets under medium(+/– 60 min) schedule flexibility. Results from the primaryscenarios corresponding to the same siding infrastructure areoverlaid for comparison. EB = eastbound; WB = westbound.

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lower average train delay. Expanding the comparison,the MB 2 3 10 EB Fleets scenario produces lower aver-age train delay than all four of the primary scenarios,including the MB 4 3 20 Fleets case (Figure 3). One rea-son is that when a fleet of trains must stop, the followingtrain in the fleet incurs delay reacting to the braking andacceleration of the leading train. The two trains cannotoperate perfectly in sync, introducing an inefficiency thatis removed if the lower-priority trains operate indepen-dently as single trains. Therefore, in cases where trafficin one direction has significantly higher priority thanopposing traffic, it may be an effective delay-reductionstrategy to invest in moving blocks enabling fleetedtrains in the higher-priority direction while utilizing exist-ing siding infrastructure.

Conclusions and Future Work

Past investments in PTC by North American railroadsmay soon be leveraged to implement advanced PTC withmoving blocks. Moving blocks promise to reduce theminimum separation required between trains, allowingfor new operational strategies such as fleeting trainsthrough a corridor. When supported by track infrastruc-ture that can accommodate train fleets, fleeting strategiesthat take advantage of the shorter headways made possi-ble by moving blocks can improve corridor operationalefficiency by reducing delays associated with the trainmeet process.

Results from RTC simulations indicate that fleetingtrains under moving blocks at low levels of schedule flex-ibility can reduce overall average train delay for hetero-geneous freight traffic. Fleeting trains is most effectivewhen paired with sidings that can accommodate fullfleets. With increasing schedule flexibility and increasingtraffic homogeneity, implementing moving blocks with-out fleets can be a more effective strategy depending onthe specific traffic mix. Low priority trains, such as unitcoal trains, benefit the most from investments in movingblocks or fleeting since such improvements allow trainsto spend less time waiting for higher-priority trains topass during a meet. High priority traffic, such as pre-mium intermodal trains, see minimal delay benefit sincethey are already given preference by dispatchers regard-less of infrastructure configuration or control system.However, such treatment can come at the expense ofhigh delays imposed on the lower-priority traffic. Whensupported by investments in moving blocks, alternativestrategies such as operating trains in fleets in one direc-tion can produce average train delay values even lowerthan operating trains individually or in fleets in bothdirections. Such a strategy could utilize existing sidinginfrastructure and be implemented on corridors wheretraffic in one direction has higher priority.

Future work should conduct further simulations withdifferent traffic mixes incorporating passenger trains andother types of freight traffic across a range of daily trainvolumes. Additional simulations would also help validatethe effectiveness of the operating and infrastructure stra-tegies presented in this work. Future work should alsoexamine mainline corridor performance under virtualblock control systems or with fleeted high priority trainsfor different traffic compositions.

While moving blocks and fleeting can produce signifi-cant delay reductions over the baseline scenario with nofleets and fixed blocks, in all other comparisons, the dif-ferences in average train delay only range between 5 and10min per 100 train-miles. While it is possible to improvecapacity incrementally through control system and trainplan changes, track infrastructure imposes a fixed limiton the benefits of such improvements. With train fleeting,moving blocks, or both, short-term capacity improve-ments can lengthen the required timeline for expendingscarce resources on track infrastructure construction suchas additional passing sidings and double track.Understanding the feasibility and effectiveness of fleetingstrategies under conventional fixed block control systemsand advanced PTC systems with moving blocks, whilemaintaining a similar amount of track infrastructure andsubject to operational schedule flexibility, can assist rail-road practitioners developing long-term capital plans,train operating strategies, or methods for improvingshort-term capacity on single-track corridors. The resultsof this research can also inform railway operators consid-ering investments in advanced control systems incorpor-ating moving blocks. While the experiments conducted inthis work are intended to be representative of NorthAmerican operating environments and train characteris-tics, the positive results for scenarios with low levels ofschedule flexibility indicate the general approach canbe applicable to railways in Europe, Asia, or other loca-tions where structured schedules for freight trains arestandard.

Acknowledgments

The authors thank Eric Wilson of Berkeley SimulationSoftware, LLC for the use of Rail Traffic Controller simulationsoftware and technical support. The authors also acknowledgeundergraduate researchers Matthew Parkes and Daniel Holmesof the University of Illinois for assistance performing the simu-lation experiments.

Author Contributions

The authors confirm contribution to the paper as follows: studyconception and design: Adrian Diaz de Rivera, Tyler Dick,Leonel Evans; data collection: Adrian Diaz de Rivera, LeonelEvans; analysis and interpretation of results: Adrian Diazde Rivera, Tyler Dick, Leonel Evans; draft manuscript pre-paration: Adrian Diaz de Rivera, Tyler Dick. All authors

Rivera et al 155

reviewed the results and approved the final version of themanuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest withrespect to the research, authorship, and/or publication of thisarticle.

Funding

The author(s) disclosed receipt of the following financial sup-port for the research, authorship, and/or publication of thisarticle: This research was supported by the National UniversityRail Center (NURail), a U.S. Department of TransportationOST Tier 1 University Transportation Center, the Associationof American Railroads, and the CN Research Fellowship inRailroad Engineering.

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