Re: [SystemSafety] Lac-Megantic disaster

From: Peter Bernard Ladkin < >
Date: Fri, 12 Jul 2013 09:53:09 +0200


For a longer, more detailed account of some features of the Westinghouse-derived air brake system, people might like to look at https://en.wikipedia.org/wiki/Railway_air_brake and http://www.railway-technical.com/air-brakes.shtml

Some people are likely comparing mentally with a simple vacuum-brake system. The Westinghouse-type air brake system has the advantages during use (to brake a running train), amongst others, of * being more responsive (for example, differential air pressure may be chosen by the designer for most effective mechanical properties, whereas vacuum is limited to 1 bar differential, obviously); * more effectively checked and maintained (it is easier to notice or find a leak in a positive-pressure line, for example by walking past it, than it is to find a leak into a vacuum line. Most train-dispatch procedures in the US according to Wiki involve a walk-past of each car to ensure air brakes are functioning. This would be much less effective or simple with vacuum-operated brakes);
* lighter (lower-pressure-differential systems are generally heavier than higher-pressure counterparts in all sorts of engineering contexts).

Stopping a running train effectively is - I would say obviously - a different problem from maintaining a stationary (parked) train stationary. The first changes the dynamics of a situation and there are effects such as take-up of slack (conversely, slack increase on release-of-braking command) which make the dynamic situation quite complicated, I should think, and the system issues are thus much wider than a single wagon. Whereas the latter only involves keeping each individual wagon where it is, and thus invites a unit solution. I would guess that having separate systems for each task eliminates the need for compromise, keeps the reliability issues separate, and likely reduces technical complexity.

The question which arises with the Lac-Megantic disaster is why both systems should be considered needed to maintain the parked train stationary. The answer is presumably: the train is parked on a gradient. Another answer may be that an engineer finds it easier to leave an engine running to power the air brake system than to go down the train setting individual hand brakes. Indeed, this second answer has been hinted at publicly by the head of the railroad company involved. We'll see what transpires with that.

I'd be keen on hearing thoughts on the practicality of a requirement to park trains only on level ground in NA. That would eliminate the risk of "migration to the boundary" procedural issues with engineers trying to save themselves a bit of work as suggested above.

Obviously, if you have a longish train, bits of it might be on level track and bits not, and it might be hard to tell (one might have to survey and signpost parking spots with allowed train lengths and position). I read somewhere that the gradient between Nantes and Lac-Megantic is 1.2%. Can anyone confirm?

PBL On 7/12/13 12:51 AM, Matthew Squair wrote:
> Like all systems there's an operational context that goes hand in hand with safe operations. In this
> case the system is designed so that if you lose pressure, due to a burst hose say, the system will
> 'fail safe' and stop the train. Once stopped you can apply the park brakes to hold you on the grade.
>
> The pneumatic train brakes are not (emphasise not) designed to be used as a means of holding the
> train while it's parked. Just like your car, there's a mechanical park brake. And just like your car
> you shouldn't hop out and walk off without applying the park brakes.

Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany Tel+msg +49 (0)521 880 7319 www.rvs.uni-bielefeld.de



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systemsafety_at_xxxxxx Received on Fri Jul 12 2013 - 09:53:21 CEST

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