27 September 2005

What leaves mean to trains

I've blogged before about the golden rules for writing press releases, but I've just been sent one that breaks one of the rules (Try to avoid writing anything longer than a page) ... and gets away with it.

Virgin Trains have just sent out a press release about how they are using washing up liquid to simulate mouldy leaves on the track so they can train their drivers (see story here), but it is a lengthy release, not least because there are several paragraphs of notes. Normally the longer the release, the less readable/interesting it is.

Not this time. For once it is interesting background info. (which I can't use in the news item, so I'll share it here!)

I never really understood what it was about 'leaves on the line' that causes so many rail delays each autumn. I'm mean, obviously it makes it slippy and that must cause trains to take longer to get going and to stop. Maybe the occasional train slides through a red light and that upsets schedules. But it can't add up to more than a slight reduction in efficiency - why do the railway companies get so steamed up about it?

Well the answer is, because it can be a bit more serious than that...

Effect of 'leaves on the line' on trains/track/signalling

In addition to delays caused by train drivers having to drive more cautiously when approaching signals and stations (known as 'defensive driving'), there are three specific problems leaves cause to the infrastructure:-When the train tries to stop, the wheels can lock and the train could slide through a station. The sliding wears a flat spot on the train wheels (known in the trade as a 'flat'). When the train gets going again the 'flats' hit the track with the force of a sledgehammer, causing potential damage that can lead to broken rails and possible derailments. It also results in the train having to be taken out of service to allow the wheels to be re-profiled. When a train tries to pull away from a station, particularly on an incline, the wheels can spin. This can lead to shallow dips (known as 'wheel burns') on the railhead. When subsequent trains travel over the 'burn' they have a similar effect on the rail as 'flats' and can cause damage leading to broken rails.

Every signal box has an electronic diagram of the track and signals it controls. Signallers know where every train is by the movement of lights across the diagram. The movement of train along the track causes a short circuit of a very low voltage charge in the track, thus illuminating its position on the signaller's diagram. (This is how a track circuit works.) Leaves on the line can interfere with this process, causing the lights not to illuminate on the signaller's diagram; hence the signaller 'loses' his trains and no longer knows where they are. (This is known as a 'track circuit failure' or TCF.) As a consequence, every train has to stop at affected signals and contact the signaller before getting permission to proceed, all of which adds considerable delay to the train.


There! Now while you are waiting on a windswept platform for a delayed rail service, you'll have a better understanding of why!

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