International Technical Committee Topic 51 Published in IRSE News | Issue 256 | June 2019 1

Prepared on behalf of the International Technical Committee by Rod Muttram

What constitutes good and acceptable practice in light rail signalling?

The IRSE’s International Technical Committee (ITC) provides a multi-national and independent perspective on Railway Control, Command and Signalling (CCS) topics. Membership of the ITC comprises industry experts from both suppliers and operators, drawn from countries around the world. It aims to inform and educate both IRSE members and the train control and communications community worldwide, principally by the production of reports on selected topics. In this issue of IRSE News we have two reports from the ITC, demonstrating the breadth of the work they carry out.

After the decline and closure of many tram systems in the middle years of the 20th Century, recent decades have seen increased interest in, and the deployment of, light rail (or rapid) transit (LRT) systems around the world to provide

higher passenger-carrying capacity and lower emissions than buses without the expense of heavy rail/metro systems.

So what do we mean by ‘light rail’ in this context? The UK ORR defines ‘Light Rail’ as follows:

“Light rail is an urban rail transportation system that uses electric-powered rail cars along exclusive rights-of-way at ground level, on aerial structures, in tunnels, or occasionally in streets. The operation is under full signal control and the current UK systems have full automatic train protection.

As the name suggests, the term light refers to operations carried out under a less rigorous set of regulations, using lighter equipment at lower speeds than those used by heavy rail, such as services provided by train operating companies.

A tram system, tramway or tram is a railway on which streetcars or trolleys run. It is typically built at street level,

sharing roads with traffic, but may include private rights of way especially in newer light rail systems.

Many older tram systems do not have platforms, which enables integration with other forms of transport and pedestrians making simultaneous use of the streets”.

The ITC finds these definitions somewhat unsatisfactory in that the distinction between ’Trams’ and ‘Light Rail systems’ is not clearly made, indeed it even talks about ‘newer light rail systems’ in the paragraph about tram systems. In our view this matters because it is misleading; the first paragraph says, “The operation is under full signal control and the current UK systems have full automatic train protection”. Mixing the terms, without saying what is expected of tram systems specifically, creates the impression that they have a level of protection that in most cases they clearly do not. Better definitions developed by IRSE past-president Clive Kessell are given in the panel below.

Metros

∞ ‘Heavy Rail’ mass transit in city centres and out to suburbs.

∞ Dedicated track, often underground. ∞ High capacity, frequent train intervals. ∞ Power supply usually 3rd or 4th Rail. ∞ Usually long trains, up to 12 cars. ∞ Mandated automatic train protection. ∞ Often automatic, sometimes

driverless, using communications-based train control (CBTC).

Light Rail

∞ A development of the past 30 years ∞ Dedicated tracks usually

at ground level. ∞ Often built on ‘stilts’ as

elevated railway. ∞ Cheaper construction for suburbs. ∞ Often a ‘take over’ of former main

line rail lines. ∞ Power from overhead wires or 3rd rail. ∞ Lightweight trains up to

three units of two cars. ∞ Usually automatic

operation with CBTC.

Trams

∞ A resurgence of 19th Century transport.

∞ Combination of dedicated track (often former rail lines) and street running.

∞ Limited signalling for junctions and road Intersections but mainly ‘drive on sight’.

∞ Sharp curves and steep gradients allowed.

∞ Overhead power supply. ∞ Single unit articulated vehicles,

sometimes with several sections. ∞ Current generation trams often ‘low

floor’ to facilitate passenger access.

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The reality is that trams are generally driven on ‘line-of-sight’, with drivers expected to drive at a speed which will enable them to stop the tram in the distance that they can see ahead, like the drivers of road vehicles. Light-rail on the other hand tends to have more sophisticated signalling and control systems similar to those found in the metro domain. There are systems which mix both operating modes and that introduces certain risks.

This article was prompted by the derailment on the Croydon Tramlink, UK, on 9 November 2016 in which seven people died and over 60 were injured when a tram overturned due to entering a curve with a severe speed restriction at too high a speed. Trams differ from buses in several ways and one of the key differences is in the consequences and potential mitigations if a curve is approached at too high a speed. A bus has the option to ‘steer away’ if an alternate route is clear avoiding harm; a tram’s route is completely constrained (rail is a ‘one degree of freedom’ system) and even with secondary braking devices a steel wheeled tram will generally not match rubber-tyred road vehicle braking distances. Thus, if the speed exceeds a certain threshold approaching or within a curve it will inevitably overturn or at least de-rail.

The ITC therefore has similar concerns regarding the over-reliance on fallible human drivers for speed control as it has for main line railways. Our chair presented on this at the IRSE Convention in Dallas in 2017 (see irse.info/itc43)

The UK Rail Accident Investigation Board (RAIB) report into the Croydon accident (irse.info/nzyec) third recommendation was that “UK tram operators, owners and infrastructure managers should work

together to review, develop, and provide a programme for installing suitable measures to automatically reduce tram speeds if they approach higher risk locations at speeds which could result in derailment or overturning”. The ITC is somewhat surprised that this makes no mention of targeting these measures to be cost effective in the way that target cost was a key part of the specification and development of TPWS for the main line railway in the UK. A low cost system that reduces the risk will most likely deliver a lot more benefit than one that effectively eliminates it but represents gross disproportion in terms of cost and therefore achieves only limited deployment.

Whilst the severity and nature of the Croydon accident made it inevitable that Transport for London (TfL) would implement some form of speed control on the Tramlink, whether other operators do so will be highly sensitive to system cost and that will be driven by system complexity and the level of safety integrity demanded.

An international perspectiveLRT systems are being implemented in many different forms around the world. At one end of the spectrum are driverless systems operating on exclusive rights-of-way that can be at-grade, underground, or elevated. At the other end of the LRT spectrum are manually driven systems, operating at-grade, that share the right-of-way with other road traffic users. In addition, we increasingly see examples of LRT systems with a mix of both dedicated and shared-use rights-of-way.

A good example of a mixed system is Metro do Porto in Portugal which includes street running, dedicated alignment (some of it along old heavy

rail routings), an 8km in-tunnel section and a 100km/h tram train service to Póvoa de Varzim in the north all integrated into a single network. Other examples of LRT systems operating in ATO on sections of dedicated (grade-separated) alignments and operating manually on sections of shared-use (street-running) alignment would include the new Eglinton Crosstown Line in Toronto, Canada and the Red Line in Tel Aviv, Israel.

All of these LRT applications have to consider the risk of collisions (as a result of inadequate safe train separation assurance) and the risk of derailments (as a result of inadequate interlocking protection and/or inadequate overspeed protection). Risk levels will vary depending on the level and type of service being provided and the nature of the right-of-way. Risk levels can be different in sections of the right-of-way with differing characteristics. These risks can be mitigated through fixed block or moving block signalling systems, simpler control equipment or through reliance on operating procedures alone.

For some LRT applications, a conservative (but more expensive) approach is taken to install the same signalling system everywhere, as dictated by the highest risk section. In other LRT applications, it is argued that no signalling system is necessary (with associated cost-savings) on the basis that the LRT is simply being operated as a ‘bus on rails’. A sample of different systems provided by ITC members is shown in Table 1.

Examination of Table 1 emphasises an issue; what is considered as ‘light rail’ covers a wide range of system level options. At one end of the spectrum a full control system applied over the whole line clearly poses no safety

The accident on the UK’s Croydon Tramlink system in 2016 resulted from overspeeding into a sharp bend.

Photo Crown Copyright, from the RAIB report into the accident.

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System Key Characteristics Length Era Control technology

Tampere raitiotie (Tampere tram) Finland

20% street, 80% dedicated. 70km/h.

15km then 26km. 2012 Lineside signals with PSR enforcement.

Metro de Malaga Spain

20% street running, 80% dedicated underground. 50km/h street, 70km/h dedicated.

11.3km July 2014 Alstom Urbalis.

ATO/ATP in tunnel section. Line of sight (LOS) with speed supervision only in street running.

Manchester Metrolink UK

33% street running, 67% dedicated alignment. 80km/h.

92km 1992 with extensions in 1998 and 2011-14

Manual LOS, Trams signals and point indicators. PSR by procedure, extended warnings.

Tyne and Wear Metro UK

5% dedicated underground, 78% dedicated, 17% shared running with heavy rail. 80km/h.

77km 1980, with extensions in 1992 and 2002.

Lineside signals with Indusi train stops. Mixed with TPWS for heavy rail trains on shared sections. Main line style approach locking.

Erasmuslijn Netherlands

Segregated. 5.9km 2006 Colour light signals and ZUB222 ATP.

Hoekse Lijn Netherlands

Segregated. Speed 100km/h.

24km with 2km extension planned

2018 Full ATP (BT CF150).

Sneltram Utrecht Nieuwegein (sun) Netherlands

Street and segregated. 80km/h.

1983 Colour light signalling based on axle counter blocks. Some level crossings (AHB).

Metro do Porto Portugal

Street and segregated and tunnel. Tram train high speed service to Pova.

Circa 67km (five lines with common core).

2002 Bombardier Citiflo 250 balise based ATP.

Al Sufouh Dubai

Mostly on-street, some elevated. 50km/h.

15km 2014 Alstom Urbalis CBTC.

Qatar tramway Street, tunnel and elevated. 50km/h.

55km (four lines) Planned 2019 Alstom Urbalis CBTC.

Reims France

Street running. 50km/h.

11km (two lines) 2011 Lineside signals with priority at road crossings.

Regional traffic Bern Solothurn (RBS) Switzerland

Dedicated alignment and underground. Track gauge 1000mm. 90km/h.

45km 1912 Lineside signals with continuous ATP (ZSL90).

Eglington Crosstown Line Toronto, Canada

Dedicated (underground) and street running.

10.2km underground 9.5km street running.

Planned 2021 Bombardier Citiflow 650 CBTC synchronised with road signals. Mix of GoA 1, 2 and 4 depending on area.

Confederation Line Ottawa, Canada

Dedicated underground and surface.

2.5km 2019 Thales Seltrac CBTC. Mix of GoA 2 and 4 depending on area.

Table 1 – Information on a selection of LRT lines worldwide. All of the examples are steel-wheeled and standard gauge, unless noted otherwise. Note that Japan also has three lines it considers ‘light rail’ but these are closer in nature to a metro system. Two are fully automatic (UTO) and the other has full ATP.

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concerns. Where no system or only simple lineside signals are applied but system characteristics pose additional risks then problems could arise, and as always, change can introduce risks. This goes to emphasise the need to be more disciplined in defining what is light rail and what is a tram.

DiscussionAs the title of this paper states the ITC’s intention was to survey a range of these systems to determine what constitutes ‘good practice’ and hopefully recommend what should be considered the minimum standard of control system to be applied. In practice the diversity of such systems and the blurred boundaries between trams, light railways and the bottom end of metros and automated people movers makes that quite difficult.

So, to offer an informed opinion of what the minimum standard should be means going back to an assessment of the risks in such systems.

In common with other rail systems the most significant ‘top level hazards’ are collision and derailment, particularly derailments at a speed or of a nature that may lead to vehicles overturning. So called ‘second generation’ trams are normally fitted with enhanced braking devices such as track brakes which give them stopping distances similar to if not the same as equivalently sized road vehicles. Treating trams as equivalent to a ‘bus on rails’ in terms of SPAD and collision risk may thus be considered not wholly unreasonable. Tram track structures often use spring loaded and trailable points such that controlled and facing points are relatively rare. ‘Routing error’ derailments are thus also likely

to be rare and largely confined to low speed areas. That brings us back to speed related derailments on curves of a radius that it is not safe to transit at the maximum system line-speed; very much the scenario that applied in the case of the Croydon derailment and at least two others on other systems within the last year. Whilst there are many other contributing factors particularly if there are tunnels and/or elevated sections this is the key risk issue for simpler tram systems.

It seems to us that having a tight curve at the end of a long straight section (even without the potential disorientation of a tunnel at night) such that failure to control speed will result in derailment is a wholly foreseeable accident which, given the human propensity for distraction, and or other loss of attention, demands some form of automatic control or automatic warning not just lineside signs. Such speed control and/or warning systems exist, so the only possible argument against fitment can be that the cost is grossly disproportionate to the benefit.

We were therefore somewhat surprised that Recommendation 3 of the RAIB report into the Croydon derailment, regarding speed enforcement, was not more strongly worded to create the expectation that a cost-effective solution (and not just a solution) should be found.

It is the ITC’s belief that any track layout which includes curves which it is possible to approach at above the derailment speed should be protected by some form of ‘speed trap’, ‘speed control’, or at the very least, a very marked audible or ‘unmissable’ visual warning in the cab if the tram is approaching at excess speed to reduce the risk of derailment

(‘Speed trap’ is the term used for the main line Train Protection and Warning System [TPWS] in GB to describe a device which measures the speed of the train at a single location and enforces a brake application if a defined speed is exceeded). This will still leave many tram systems and parts of tram systems where ‘line of sight’ driving is permissible without such controls because the curve radii are such that an overturn is extremely unlikely. Thus, for most trams, a truly intermittent ‘TPWS like’ device seems likely to be the most cost-effective solution.

The cost conundrumIt seems to the ITC that the big problem with the application of control systems to trams has been an element of ‘the best being the enemy of the good’. The number of new tram systems being built represents relatively low volume so in many cases the fitting of control systems that are (perhaps slightly modified) versions of heavy metro solutions as a ‘dedicated’ development cannot be justified. If the owner and operator are prepared to pay the up-front and on-going maintenance costs for such a system then from a safety perspective this is good, but many tram systems are very budget dependent in terms of whether they get built at all, and reference to the risks above would indicate that such a solution represents ‘gold plating’ for many trams that are mostly street running and/or at grade tracks with large radius curves.

We also recognise that trams are at the ‘sharp end’ of competition with road and that the factors that led to a revival could reverse with the increasing adoption of autonomous and zero emission

A modern low-floor light rail vehicle, in this case a Bombardier vehicle for the Gold Coast in Queensland, Australia.Photo Bombardier.

Heritage vehicles are in common use on densely used systems such as this example in Hong Kong.Photo Shutterstock/Glen Photo.

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road vehicles. Any guidance must remember that in terms of added costs, and in particular regarding the issue of required safety integrity and validation for tram control and signalling systems, the requirements must be addressed pragmatically.

For speed control/warning alone something much simpler than the existing systems should be possible and if it provides an underlying monitoring and intervention/warning function normally unseen to the driver it should be possible to avoid needing a high level of safety integrity with its associated high validation and approval costs.

Many systems deployed on trams, even those purporting to be tailored or specifically designed for the application are advertised as being designed and validated to CENELEC SIL 4. But if we consider a truly intermittent ‘TPWS like’ speed trap which intervenes only if the approach speed to a restricted curve is too fast then this is an ‘on-demand’ or ‘low demand mode’ function under the core IEC61508 standard. That is a much less onerous design requirement and it should be possible to meet it with a single channel ‘commercial off the shelf’ solution.

It also seems entirely feasible that a ‘no SIL’ system that measures speed continually and aligns this to the geographic whereabouts of a moving tram but has no interface to the tram braking system, merely giving an urgent vigilance alert to the driver, could well be an acceptable solution in terms of reducing this risk ALARP. This should be the very minimum that would be acceptable to the safety authorities beyond the provision of additional or re-positioning of fixed lineside signs for layouts with this type of derailment risk.

So the challenge to those implementing recommendation 3 should be to come up with a ‘cheap and cheerful’ intermittent speed control or warning solution. The challenge to the regulators is to accept the deployment of something that significantly reduces risk, even if it does not meet the ‘normally expected’ standard of integrity for main line or heavy metro railway signalling. Such a solution would then be available as a cost-effective risk reduction for other systems with similar derailment risks.

Current status and conclusionTransport for London has made good progress on a number of the RAIB

recommendations including improving situational awareness for drivers and managing driver alertness. Temporary lighting has been installed on the approach to Sandilands Tunnel and tunnel lighting is planned for this year. Additional speed signs have been added and a network wide maximum speed reduction from 80km/h to 70km/h implemented. An innovative new ‘Driver Protection Device’ has been installed which detects and manages fatigue and distraction. That system uses advanced, safety-verified sensors that track eyelid closures and head movements so that when fatigue or distraction is detected an in-cab alarm is sounded and the driver’s seat vibrates to refocus the driver’s attention. The new iTram information system planned, based on proven technology from the bus industry, will provide an in-cab alert if the speed limit at any location is exceeded.

Despite the unquestionable reduction in risk that the above measures will have brought, TfL is still (perhaps unsurprisingly) responding to recommendation 3 by installing an automatic speed control system. The contract has now been placed with ESG (a DB company) with the main component parts of the system supplied by Sella Controls and their sub-contractor EKE of Finland.

For a full description of the system please see Clive Kessell’s excellent article in the April 2019 edition of Rail Engineer (irse.info/86m4s) but in summary it is, as we postulated above, a ‘TPWS like’ speed trap which will be applied at the vulnerable locations and directions (some curves are only a risk from one direction of approach). Like main line TPWS, it has its origins in a right-side door enable system but by using more modern communications technology and digital messaging needs only one beacon per ‘speed trap’ operating in the unlicensed 865.7 to 867.9 MHz band, rather than the two low frequency analogue loops of main line TPWS. The positioning of the beacons is not critical although they do need to be known as the system uses them as Absolute Position References (APRs) or Norming Points (NPs) like many metro CBTC solutions to accurately locate tram position. The beacons are ‘telepowered’ by the passing tram and answer back with the start and end of a zone and the maximum speed allowed within it. An on-board computer then uses the existing odometry to measure to the start of the zone and will initiate a

brake application if the speed goes above the permitted maximum at any point within it. Graduated speed reductions can be enforced using multiple beacons, which can be placed together or separated depending on need and other track features since the distance to the zone start is calibrated but not determined by the beacon position. We understand that for Croydon between 2 and 4 beacons will be used at each ‘control zone’ depending on location. The on-board computer automatically communicates an incorrect beacon sequence to a central control station if a beacon is faulty or missing (the latter by knowing the sequence of beacons in a route) via public 4G. Should an over speed be detected, then the brakes are automatically applied and the tram is brought to a stop. It is understood that there will be an override which will allow a driver to reset the system and proceed (but only after getting permission from control) in the event of either a trip or a system fault.

In the opinion of the ITC there is little doubt that technically this system will do the job and reduce what is already a very low risk firmly into the ‘broadly acceptable’ band. The key question as far as we are concerned is what is it going to cost? At present both TfL and ESG are being ‘tight lipped’ about the contract value so we have little idea of the recurring and non-recurring costs and which of the non-recurring costs are true ‘one-off’s’ and which are Croydon specific. In our view that will be critical in determining whether the system is more widely adopted or remains unique to Croydon. The system has been specified by the client as ‘SIL 2’ which is undoubtedly ‘overkill’ at the level of risk involved for this ‘on-demand’ system. However, it is clearly necessary for the system to have some defined integrity to allow an overall case to be made and it is likely that specifying SIL1 or SIL0 would not save much, particularly if the system components have already been assessed to SIL2 for other applications.

The ITC is very supportive of this project, which looks like a pragmatic technical solution given the circumstances of something having to be adopted. If the costs can be controlled to a reasonable level, wider adoption by other tram operators seems likely. Since this is a public procurement contract, sooner or later the contract value and its make up should eventually emerge. As and when that happens, we will issue an addendum to this article.