An update on the Cleveland street over run

17/06/2014 — Leave a comment

Cleveland street train overrun (Image source: ATSB)

The final ATSB report, sloppy and flawed

The ATSB has released it’s final report into the Cleveland street overrun and it’s disappointing, at least when it comes to how and why a buffer stop that actually materially contributed to an overrun came to be installed at Cleveland street station. I wasn’t greatly impressed by their preliminary report and asked some questions of the ATSB at the time (their response was polite but not terribly forthcoming) so I decided to see what the final report was like before sitting in judgement.

Here’s what the ATSB said about the buffer stop design.

A buffer stop’s capacity to stop a train is determined according to the train’s maximum weight and speed. When designing buffer stops a standard method is to base calculations on a passenger train travelling between 10 km/h and 15 km/h. For example, for the design of buffer stops on terminal tracks, German passenger train operator Deutsche Bahn uses a collision speed of 10 km/h in their calculations.

ATSB final Cleveland St report RO-2013-005

Sounds reasonable, especially that bit about what the Deutsche Bahn use as a collision speed. But upon closer examination it all starts to look less than convincing. First off let’s think about the statement that the Deutsche Bahn uses a 10 kmph limit for their buffer stops, this is known as an appeal to authority type argument, but in practice we know nothing about the context in which DB might apply a 10 kmph impact design rule. For example is it a terminal bay fitted with end impact walls that would prevent a train overrun? Or a low platform with no end impact walls to speak of? Is it a minimum figure? Or an absolute value? In practice quoting magic numbers like this without any context is meaningless. Oh and both the DB 10 kmph number and the 10-15 kmph ‘standard method’ number quoted are un-referenced to any source, which is just plain sloppy.

In contrast the UK standard on such things GC/RT5033 states that 10 kmph is to be considered an absolute minimum performance requirement, GC/RT5033 also goes onto state that the designers of a buffer stop need to consider all the relevant factors including the energy/speed of impact and what happens if the train overruns. So clearly there’s a little more to the design of buffer stops than just plucking a design out of a catalog to meet a magic 10 kmph value, in fact a risk assessment needs to be carried out and turning once more to  GC/RT5033 that standard does require such an assessment be conducted and also that, if necessary, buffer stops be augmented by end impact walls (1).

For tracks serving passenger traffic, the end impact walls should be designed for a horizontal design force of 5000 kN at a height of 1.0 m above the top of the rail where a buffer stop with a minimum braking capacity of 2500 kNm is provided.

Rail Group Standard GC/RT5033

You see the fundamental error the authors of the ATSB report make is failing to recognise that while buffers serve the purpose of stopping an over-running train without injury to personnel this is not the same as preventing the train from over-running into a protected zone such as a railway concourse, for which an end impact barrier is the go to design solution. In other words when there’s concourse full of people at risk behind the buffer stop you need to consider the possibility of a train overrun into that space regardless of how well you think the buffer stop is going to work.

There’s also no discussion of how the buffers inherent design made it likely that a train coming in above it’s design limit would ride up and over the top, essentially failing deadly and assuring that the train would end up at concourse level. Which proves the point made in the previous paragraph that stopping a train from speed is a separate safety function too preventing a train from over-running a concourse.

Finally there’s the question of whether this is a specific one off problem or whether Queensland Rail has a more general problem across all of it’s station platforms. You would think that a logical and conservative conclusion would be that where there’s one design error there well be more and that the ATSB would recommend a general network review of all station concourses for over-running risk in light of this. Well you’d be wrong. Instead Queensland Rail just upgraded that specific buffer stop to handle a 255 tonne train moving at 15 kmph. Not to handle a 256.5 tonne train moving at 30 kmph (2), as happened at Cleveland Street I hear you ask? No. Which means that the new buffer would still fail if the accident were to happen today.

Worse yet, neither the ATSB or Queensland Rail acknowledged the hazardous failure mode in which the buffer failed at Cleveland Street that contributed to the overrun, nor the distinct difference between stopping a train and preventing it from over-running into a concourse.

So overall I give the ATSB a big thumbs down for their effort, the analysis was flawed, their research cursory and reasoning sloppy resulting in conclusions and actions that were incomplete or ineffective. The ATSB needs to do better.

Notes

1. A structure or other arrangement located behind a buffer stop designed to contain a train that has run through the buffer stop, so preventing harm to people or damage to critical structures that would otherwise have been in the path of the train.

2. Doubling the speed quadruples the kinetic energy of the impact.

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