INTRODUCTION

The design, fabrication, installation and maintenance of S&C involves a complex array of measurements within the layout, all intended to ensure the installed and soon to be maintained article remains as close to its design as possible. Each step of the process has varying levels of intricacy ranging from new cast pieces measured to 0.01 mm accuracy right up to the assembled panel where build-up tolerances such as rail curving, drilling, cutting, rail to seat placement, centre hole placement or panel alignment affect the precision of the layout. This article aims to provide guidance to inspectors, designers, maintainers and those who are generally interested in this topic on the correct way S&C should be inspected through its intended life cycle to avoid the inevitable comment: “It didn’t look like that in the yard?” It will also discuss other factors that often get overlooked during the development of the design. The subject of switch and crossing inspection will be covered in two parts: Part 1 comprising of design and prefabrication and part 2 including installation and maintenance. The intention is to ensure there is a link between the two parts. Therefore, part 2 will summarise the full cycle.

THE KEY IS THE DESIGN

S&C configuration selection in most cases is pre-determined by track standards or by a mandated specification based on the track category. When schemes with non-generic requirements such as direct fix trackform, non-railway engineering

permanent way (REPW) designs or highly space constrained layouts are to be designed, finding a solution that best fits all its intended functions becomes difficult.

The system compatibility, reliability, maintainability and constructability all play a part in varying measures making the selection process of the design factors more difficult as technology advances. Of course, in addition, assessing the construction and operational safety of the design options is paramount. How the components are handled and replaced and the layout design itself all affect the inspection activity and are part of this decision-making process.

When problems arise on site we revert to the design. Rarely nowadays do we hear of pre-fabrications or installations going wrong in a big way. This is mainly due to the level of assurance designers and suppliers apply to make sure the manufacturers’ 1:50 drawing matches the 1:200 general arrangement and that the 1:200 works on site. Best practice involves the final design being verified or set-out to help identify errors in design or survey. By allowing for track movement between the survey date and the setting out dates, we can assure ourselves the layout will fit. When it doesn’t, we simply check to see where the error could be occurring.

We should also consider a full topographic survey of the as-fabricated layout and verify this with the design and on site. This has proven to be very effective in the past on TfL sites. Most suppliers undertake a setting out check for their own quality assurance records and have the ability to share

Switch and crossing inspection: from manufacture to maintain

AUTHOR

Darren SharpCEng ICE/FPWI Permanent Way Consultant

Darren has 30 years experience within permanent way engineering covering

design, innovation, manufacture, renewals and maintenance. He has worked for British Rail, Scott Wilson, Jarvis, Transport for London and various consultancies. Darren has worked mainly for London Underground over the last 20 years being involved with project engineering management and as the principal engineer for S&C.

Image 1: NYCT Elevated switch panel on Sekisui FFU bearers. Simple pane complex ironwork.

PART 1

Table 3: F5554 - Free switch inspection form.

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this data with the installation team to perform this check if agreed. This gives the added assurance that what was built works in the ground to a certain degree of accuracy. Not all installation contractors and S&C suppliers undertake this level of verification as a matter of course and consideration should be given to specifying the appropriate checking processes within any new tenders.

The relationship between the scheme designer (design body responsible for the initial general arrangement incorporating the alignment) and layout designer (S&C manufacturers designer) is vital to the delivery of the intended design. From time to time you hear of layouts when drafted in 1:50 using the prescribed REPW lead rail design geometry not correlating with the original 1:200 CAD design file. This occurs when true tangential geometry is used by the scheme designer against the standard secant design of REPW turnouts. The variance is increased when the through radii is tightened.Recently canted layouts and those with tight vertical geometry have also been replicated at the pre-fabrication stage. For canted switch panels this helps to prove the POE (points operating equipment) is able to achieve the required switch fit and flangeways within its operating range. For example, some point machine installations will struggle with

sufficient power on canted track above 25 mm when operating from low to high side. Obtaining a fully closed switch along the planed length and the correct flangeway are all affected by the additional drive force required on canted switches. The arrangement to cant the layout should be agreed at the design stage with the supplier.

Other construction types require differing levels of pre-fabrication requirements and inspection checks. Image 1 shows a guarded turnout panel layout for NYCT (New York City Transit). The extended bearers are for the cess walkway and structural support on the elevated structure. The use of FFU (fibre reinforced foamed urethane synthetic wood material) here is guaranteeing a level fabrication and installation when the panel is supported on the girders of the structure. The need to inspect the bearer straightness when supported on a structure is critical to achieve the correct elevation.

PRE-FABRICATION INSPECTION – CONFIGURATION DRIVEN REQUIREMENTS

This section sets out how the configuration defines the requirements for fabrication inspection measurements.

Each configuration and construction type introduces its own risks in terms of achieving the build quality required for S&C. For example, the introduction of tie plated modular units presents an additional risk that the tie plate itself is not seated centrally thus affecting gauge and possible check gauge or flange way passage (FWP). This can occur at prefabrication or on site where rail fixtures prevent any adjustment when built as part of a complex layout. This was observed on a LU (London Underground) layout where acceptable tolerances for the verticality of the end face of the bearer within the tie plate joint were not defined, resulting in a gap that was too tight and therefore only just managing to achieve the required gauge tolerances of +/-3 mm. When this occurs opposite the crossing nose the tighter tolerance of the check gauge or FWP come into play with +2. -1 mm tolerances making this very difficult to attain. Image 2 shows a complex arrangement of tie plates where check gauge can be compromised by non-centred tie plates. In this example however, the lessons learned from previous layouts ensured the modular joint on the bearer end allowed the plate to fit providing the design gauge within 1 mm.

Once the configuration is agreed the method of inspection and testing will follow, taking into consideration the build-up of the layout. For example, with non-ballasted/direct fix layouts there is no gauge or rail plane control and variance in construction height will need to be allowed for by ensuring a level layout for measurement is achieved. Table 1 lists the inspection criteria against the three main configuration types. Light green signifies a requirement to inspect and dark green indicates that the inspection is more critical.

The supplier must then ask the question; “If I take up all the tolerances in the yard, how is the installer and maintainer going to cope?” When new products are designed, the system integrator is responsible for managing these risks. Hazard identification at component, sub-assembly and full system assembly is paramount at the design selection stage to avoid the chosen option becoming un-maintainable. Undertaking a maintainability analysis to ensure all components can be restored to their full operating condition is something we tend to carry out for new products. If we were to apply this process to our current standard suite of designs, we may find ourselves rethinking the design and fabrication methods used. Using this process can help define specific issues such as the ideal location for tie plates, welds, cast shoulders and other bearer mounted furniture which affects the buildability and maintainability of the layout.

Table 2 shows various examples I have experienced that have led to discussion topics in the past and where possible, introduced improvements to the design, prefabrication or installation process.

It is all very well listing these risks along with the appropriate countermeasures to be put in place, however, the important part of the process is to communicate the issue to all users via briefings or O&M manuals. Lessons learned tend to pick up these issues where

Image 2: LU Kings Cross scissors with compact tie plate design.

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they are not envisaged at the development or design and planning stage. This also includes the manufacturers’ pre-fabrication team. The intention with this being to avoid repeating errors and the associated pain experienced by multiple contractors or organisations installing and designing S&C who have not always shared these lessons learned. Having a log capturing these issues from concept to commissioning is therefore vital.

It is common practice that the only person to have full awareness is the project engineer who is involved from concept to handover. LU

produced a switch system O&M manual for full system integration covering the point machine, mechanical supplementary drive and detector, switch panel and all associated parts such as stretcher bars and soleplates. In doing this the interactions and the responsibilities between the track and signals technicians could be clearly defined.

To measure the performance of the switch when operated under power, measurements such as flangeways, switch to stock rail fit/gap, gauge, slide to switch rail gaps and free wheel passage are taken on both normal and reverse

positions once the switches have thrown over 50 times or more. This enables any potential set up issues to be raised and also ensures the fully installed and operational POE system meets the required standards. Testing of LU layouts in 2008 highlighted the need for a test to be performed with the switch rails disconnected from stretcher bars and POE to assess their flexural performance. This test developed into the free switch test to assess the switch in its un-connected free state and whether any spring force resides within the rail due to over or under curving at manufacture. Each switch rail is barred open and closed at the drive point at the toe. Table 3 contains form F5554 that is included within London Underground standard T0435 – Prefabrication inspection of junction work. It is worth noting that this should be performed with the switch rollers not in use and on a level plated turnout. Testing of over 50 turnouts lead to the agreement of the main acceptance criteria for achieving a natural flangeway greater than 30 mm.

NON-BALLASTED/DIRECT FIX PRE-FABRICATION AND INSPECTION

In order to gain acceptance for a fabricated direct fix layout, the rails and plates need to be presented to enable full measurement, clash checking and functional testing of the switches with the points operating equipment (POE) installed and operated on power. Holding and bracing rails to their alignment and gauge to avoid movement during POE testing or whilst inspecting is critical and is dependent upon the design being used. In-street tramway (no sleepers or bearers) or direct fix layouts with no restraint from weight, tend to use jig and rack bracing as shown in image 3. These are limited in size where large layouts become difficult to pre-fabricate using this method. Breaking up the layout into individual panels can then introduce other risks where the full layout cannot be inspected in its entirety.

Define S&C configuration • Technical and system

requirement specification

1:200 and 1:50 Integrated design review• Produce inspection and

test plan

Construct and test pre-fabricated layout • Issue inspection and test

plan report

Undertake design and survey verification and gross error check • Use pre-fab survey and

issue verification report

Engineering approvalprior to Installation • Use inspection and test

plan and verification report as evidence for approval

Figure 1: Project assurance plan.

Image 4: Delkor layout supported on timber bearers for POE testing.

Image 3: Jigging systems used for in-street direct fix dual gauge turnout.

Table 1: Inspection requirements for different layout configurations.

Standard bearer layout Modular tie plated layout

Direct fix layout - single rail support

Rail or plate inclination N/A N/A YBill of materials - small parts

count Y Y YPOE positioning check N/A N/A Y

Cant N/A N/A YFixed heel squareness N/A N/A Y

Gaps in construction - voids between plates, pads etc N/A Y Y

Build up accuracy - rail plane and construction height

varianceN/A Y Y

Orientation mark up Y Y YRail support clash check N/A N/A Y

Third and fourth rail clash check Y Y Y

Free switch test Y Y Y

Prefabrication inspection criteria

Layout configuration

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Eliminating slip and trip hazards whilst inspecting the layout is also crucial for the safety of the inspections team.

For larger layouts, using temporary hardwood bearers to gauge, obtain rail plane, support and align the baseplates is a common method, though does create a high amount of wastage. This does rely upon a level floor and any difference in construction height will require varying depths of timber. In some cases, it may be decided that the temporary timber bracing is used to brace every fourth or fifth bearer prior to concrete pour. This can present problems when trying to remove post pour as it introduces cold joints that can lead to cracking. It is understood that a method in which layouts can be pre-constructed for inspection using permanent light weight bracing which remains in track is being explored at TfL. Composite plastic and concrete materials are being considered for this application and will potentially provide a huge benefit to reducing install times and obtaining the design alignment. Image 4 shows a Delkor resilient baseplate layout using temporary bearers to enable inspection.

To summarise the process for gathering the appropriate evidence to gain approval for the layout design and pre-fabrication, figure 1 details the steps to be taken and evidence to be obtained. This forms part of the project assurance plan and engineering deliverables.

FUTURE INNOVATIONS

The advances in using 3D scanning and digital twin replication will allow the layout to be digitally modelled at the prefabrication stage enabling verification of the installation to a greater accuracy and detail. Use of these technologies for maintenance inspections and corrective work to restore the physical layout to its original design will assist in prolonging the life of the asset by avoiding the loss of the true geometry and individual parts. This can make switch and crossing welding/grinding, positioning of welds/joints, turnout geometry rail wear and materials ordering far easier to manage. The 3D model contains measurable data making this a powerful BIM tool. When coupled with O&M manuals, defect history records and follow-up maintenance works undertaken, it will help to provide a holistic approach to asset management.

SUMMARY

This part 1 article has identified key inspection issues for the design and prefabrication stages of providing switches and crossings and has given appropriate guidance for how they can be addressed.

Part 2 of the article will be published in the July Journal and will cover the issues and requirements for inspection during the installation and maintenance phases.

COMPO-NENT PART OR ASSEMBLY

ISSUE OBSERVED COUNTERMEASURE

Cast crossings

• Cant measurements being taken on nose topping.

• Xing Vee baseplates installed with room within seat on field side for gauge to widen under load.

• Bearer spacing and squareness from panel lifting causing incorrect gauge within crossing.

• Gauging of S&C training provided to surveyors, engineers and installers.

• Select correct baseplate type to ensure tight fit on housing on field side to avoid gauge widening.

• Panel mark-up includes squareness string line to replicate panel squareness.

Stock and switch rail

• Incorrect drilling of distance block positions causing incorrect switch to stock fit.

• Switch rail has excessive spring or does not fit against stock rail.

• Switch rail twisted on slide plates – with and without POE fitted.

• Ball and claw not central.• Surveyors measuring gauge prior

to POE install when switch has a residual opening.

• Stock fronts not square and incorrect longitudinal positioning in relation to switch tips. Measuring squareness is not always accurate.

• asure all stock and switch drilling positions as part of inspection.

• Use LU free switch inspection test at pre-fab and install.

• Check rail inclination with feeler gauges. Layouts to be laid on level ground with no more than 2 mm slide plate gaps.

• Measure panel squareness, ball and claw gaps and settings.

• Close up switch with clamps. Revised RSO standard does not allow greater than 2mm RSO.

• Measure front and toe squareness, heel squareness, identify lead rail. Design requires better indication of this. Use squareness laser. Mark up heel square line on panel.

Distance blocks

• Incorrect distance block offset.• Switch foot fouling distance block

bolts.• Twisted distance blocks causing

switch foot to foul (old bull head design).

• Torsion control bolt not maintainer friendly.

• Measure rail offset from EOP to heel. Measure gaps between closed switch and distance blocks.

• Design of new domed bolt and reduced thickness distance block. Foot relief also amended. Inspect closely.

• Modified design with increased clearance to closed switch foot.

• Change of fastener type for maintenance shimming/replacement.

LVT Sonneville blocks

• Incorrect screw bolt used causing the block to crack.

• Tilting of slide plates blocks at pre-fab and install affecting switch fit and rail inclination.

• Switch tips sitting above slide plates post-installation.

• Added to O&M (operation and maintenance) manual and guidance briefed to staff on LVT systems.

• Blocks correctly wedged at pre-fab and jigs modified for install. Jigs trialled at pre-fab.

• Complete free switch test as part of pre-pour alignment tests. Also applied to Delkor.

Resilient direct fix baseplates

• Heel plate position not fixed affecting heel angle offset and squareness.

• Modify design to include a rail anchor to fix first heel plate position longitudinally.

Shallow depth baseplates

• Stock rail has too much lateral adjustment in slide plate seat causing gauge variation between pre-fab and install.

• Amend design for slide plate to include reduced rail seat area and include e-Plus clip to reduce rail role and gauge variation.

Concrete bearers

• Heel bearers out of square or wrong longitudinal position causing switch heel angle to be incorrect.

• LU design change introducing anchor plate at first heel plate on all designs.

Modular tie plates

• Coil washers not installed causing premature failure of bolt.

• Incorrect bolt length installed causing bearer cracking.

• Incorrect torque applied causing joint or concrete to fail.

• Bearer end pads not installed.• Bearers skewed or stepped at install.• Design placement of tie plates at

joints or locations where twists can occur, avoid using short single rail bearers.

• Track standard did not specify how many missing or defective screw bolts are permitted.

• Most issues here were dealt with by producing an O&M manual and briefing note to staff.

• Design guidance issued to avoid plates being installed at joints and avoiding the use of short single rail support bearers.

• Track standard S1158 updated to reflect this requirement under maintenance inspection.

Conductor rail

• Insufficient room or clashes for insulator pot drillings or slipper runs

• Incorrect offset or alignment of insulator pot holes.

• Pot hole detail and clash checks performed at 1:50 using assembled parts. Slipper run support holes also included.

POE • POE and soleplate orientation.• Hand of directly driven rail (Surelock)• MSD crank handing. • Stretcher bar to switch rail bolts.

• Issues with handing of insulation and rail potential addressed under bulletin and drawing updates.

• Changed to Hardlock to avoid inconsistent supply.

Table 2.

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TECHNICAL ARTICLE

AS PUBLISHED IN

The PWI Journal April 2020VOLUME 138 PART 2

If you would like to reproduce this article, please contact:

Kerrie IllsleyJOURNAL PRODUCTION EDITORPermanent Way Institutionkerrie.illsley@thepwi.org

PLEASE NOTE: Every care is taken in the preparation of this publication, but the PWI cannot be held responsible for the claims of contributors nor for the accuracy of the contents, or any consequence thereof.

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