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Mind the gap: keeping trains and platforms level is a 'complex' issue

David Shirres

Raised areas on platforms provide level boarding for passengers
Raised areas on platforms provide level boarding for passengers

Britain’s rail and Tube passengers get on and off trains six billion times each year and very occasionally have accidents as they do so.

Moreover, while there has been significant investment in the provision of lifts and ramps, it is, in most cases, impossible for mobility-impaired passengers to get from the platform onto a train without assistance.

It might be thought that it would be a relatively straightforward matter to solve such problems by providing level gap-free access between platforms and trains. However, matching trains to platforms on the 19th-century rail network is invariably problematic.

The relevant standard specifies that new platforms should be 915mm above rail height, have an offset of 730mm (horizontal distance between rail and platform edge) and should not be on a curve of less than 1,000m radius. These dimensions are a trade-off between the requirements of passengers and ensuring that platforms aren’t struck by any of the different types of trains.   

Aged infrastructure 

This requires all trains, which sway at speed, to fit within a dynamic loading gauge and for platforms to be outside this gauge. This gauge varies by route owing to the differing construction practices of the original railway builders and the type of traffic. Freight trains generally require a larger gauge than passenger coaches. 

A recent transfer to Scotland of London commuter trains highlighted this gauging issue. Before these trains could operate Scottish services, bogie packing rings were required to raise the coach bodies, and entrance steps had to be cut back. This illustrates why there has been a trend to provide go-anywhere trains although these tend to give larger vertical steps and horizontal gaps. 

The loading gauge requires greater offsets at higher platform heights. Hence a 1,115mm-high platform, the typical height of most train floors, would require a 900mm offset. This compares with the 730mm offset of platforms at the standard 915mm height which research has shown to be the optimum platform height. 

Most trains have a floor height of around 1,100mm to accommodate their bogies and underfloor equipment. Yet, as Greater Anglia and Merseyrail’s new trains show, it is possible to have a 950mm floor height which, with an extending bridging piece as the door opens, permits unassisted access for those in wheelchairs. 

Avoiding trip hazards 

In practice, it is not possible to ensure absolutely level access, for example owing to variations in suspension height as passengers board the train or minor height differences in ballasted track. A height difference of as little as 10mm can present a tripping hazard. 

Merseyrail’s trains can achieve this lower access as the coach floor at the doors is slightly lower than the floor over the bogies and has a ramp between the different floor levels. Such an arrangement is not possible if doors are above the bogies. For longer coaches, on long-distance routes, this is the normal arrangement and maximises passenger capacity.

Of all platforms on the mainline network, the current height and offset requirements are achieved for 30% and 22% respectively, with only 7% meeting both requirements. A fifth of all platforms fail to meet the curvature requirement. 

Although bringing 2,500 stations with 6,000 platforms up to current standards in one go would be prohibitively expensive, there is scope for targeted action. On the Merseyrail network, 100 platforms have been upgraded ready for the new trains. At core Thameslink stations, platform humps have been installed at fixed train locations to provide level boarding for passengers with reduced mobility. 

Clearly, matching trains to platforms is a complex issue. In an ideal world all platforms would be straight, have the same heights and offsets, have slab track, only have one type of train and it would not be necessary to consider dynamic gauge as all trains would stop at the station. In practice most stations have few of these ideal requirements. However, much is being done to find solutions that work in the real world.

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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.

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