On Breaking Trains

Or why systems need to be robust

On 22 October 1895 a steam locomotive approached its Paris terminus slightly faster than normal hoping to make up lost time. Except that rather than stopping at Gare Montparnasse, as planned, it crashed through the end of the platform, over the concourse, through the station facade and down onto Place de Renne a full storey below. Apparently, the only casualty was a woman who had the misfortune to be standing in the street and was struck by falling masonry.

 

The question arises, why did the train fail to stop? 

Following the subsequent accident inquiry the hapless locomotive driver is reported to have been fined 50 francs and sentenced to two months in gaol, because he approached the station too fast. One of the guards was fined 25 francs, because he was apparently too pre-occupied with paperwork to apply the hand brake.

One might assume that this was all there was to it, however as it turns out the driver and the guard were not solely responsible. 

It is also believed that the train’s Westinghouse air brakes had rather tragically failed, which seems to me a rather more significant event, but not for the reason that you might think.

Trains were of course a wonderful invention, which had transformed the world by making mass transit possible over long distances. The trouble with steam locomotives, at least in their infancy, was that nobody was quite sure how to stop them.

They travelled faster than anyone had travelled before, but were also big and heavy. The locomotive had a hard enough time stopping itself let alone the passenger cars and goods wagons that followed behind. It was not unusual for the following carriages to catch up with the locomotive when the brakes were applied causing them to collide, first with each other, and then with the back of the locomotive.

In order to make the train stop within a reasonable distance it was realised that brakes had to be added to the carriages too. The obvious difficulty was how to apply the brakes on the locomotive, and all the carriages, at the same time.

Initial solutions were somewhat rudimentary. In the United States a brake man sat on top of the first carriage. When the driver blew the train’s whistle he was responsible for applying the brake on the first carriage. He was then required to run down the roof before leaping onto the next carriage whereupon he applied its brake. This process was repeated until he reached the back of the train. This was, as one could imagine, a rather precarious job and not surprisingly there were many casualties.

The American entrepreneur and engineer George Westinghouse, like everyone else, saw the problem. Unlike everyone else, Westinghouse came up with a solution. He joined the carriages together with airtight hoses and used compressed air to apply the carriage brakes almost simultaneously. The system worked brilliantly, bringing trains to a halt with great effect. For many people the idea of stopping a large heavy object travelling at high speed with nothing more than air had initially seemed a little crazy. When it worked Westinghouse was rightly seen as a genius.

Except that there was a problem that no one at the time had foreseen. If there was a loss of air pressure, due to leak in the system, the breaks wouldn’t work. It is believed that this is exactly what happened at Gare Montparnasse. Understandably, fail safe systems where added to subsequent designs.

Knowing this story I was rather intrigued by the heritage steam train that I happened across while on a recent camping trip in Cumbria. In the images below you can see a red  pipe on the back of the locomotive, which passes between all the carriages and can also be seen at the tail of the last carriage. There is also a small pressure vessel beneath one of the seats in each carriage, but you can’t see that in these pictures.

   

In case you haven’t guessed I rather suspect that what we have here is a rather old fashioned air-break system not unlike those used on early locomotives. I didn’t get chance to investigate further, but I am going to assume its the mark two version.

The next question is what this has to do with a structural engineering blog? The reason I decided to write, other than the fact that I found it interesting, is the principle of robustness. An otherwise brilliant idea, which made a big difference to the safety of trains, was, in its earliest conception, flawed. It wasn’t flawed because it didn’t work. It was flawed because it was vulnerable to miss-use or accidental damage. In short it was not a robust system.

This ought to be a concept familiar to all structural engineers. The archetypal accident, at least in the UK, was the partial collapse of a 22 storey tower block in 1968. The tower stood quite happily until a gas explosion blew out one of its walls, causing the walls and floors above to collapse like a pack of cards. Unfortunately the component parts were not adequately tied together and were therefore unable to bridge over the damaged section of the structure. Four people died and 17 were injured.

The concept of robustness is not necessarily aimed at particular events or circumstances, rather it is intended to provide a degree of resilience against the unforeseen and the unknown. It now seems obvious that structures should not fail the moment the design load case has been exceeded, but it was not always so. 

Of course a supplementary question one might ask is how robust does a structure need to be? How robust is enough? That’s a difficult question to answer, but an ingenuous formulation has been devised, which has come to be known as the principal of ‘disproportionate collapse’. Put simply this means that any damage suffered by a building should not be disproportionate to the event that caused it.

So what is considered proportionate? That’s a rather big question, which perhaps needs its own post at some future point.