Technical signalling question

[editorsfoot]

On my way back from Acton I took the following photo:

http://inspectorsands.fotopic.net/p35331115.html

The train is held at the red signal that can just be seen the embankment, the next red signal is on the end of the platform.

As a matter of interest how close could the train behind come, as some people seem a bit annoyed that the train could sit just outside the station like that!

[tubeguru]

On my way back from Acton I took the following photo:

http://inspectorsands.fotopic.net/p35331115.html

The train is held at the red signal that can just be seen the embankment, the next red signal is on the end of the platform.

As a matter of interest how close could the train behind come, as some people seem a bit annoyed that the train could sit just outside the station like that!

On the face of it, it does seem pointless for the train to proceed from the platform, as the driver can see the distant signal (which he is now stopped at) is red.

The key issue here is that of the "overlaps". As you can see, there is little room beyond the starter signal and the rear of the train here - not enough for a train SPADDING the starter signal to stop before colliding with the rear of the train if travelling at the maximum speed for that section of track (20-25mph I'd imagine).

The starter signal (the one at the end of the platform here) has an "overlap" - a section of track which the train in front MUST clear before the signal will clear to a proceed aspect. As the starter is still red, the train has not cleared the overlap.

On the Underground, it is not considered safe for a train to be displaying only one red signal behind it, especially in this situation. A following train will not be allowed into the platform here, because the overlap after the starter signal is too small.

The home signal at this station (the one behind the camera controlling the entry to the platform) will almost certainly be showing a red aspect to stop a train entering the platform and risking overrunning the starter signal and running into the back of the this train in the picture.

This is why trains are limited to 5mph through closed stations. Imagine this station is closed and there is NO 5mph speed limit. A train approaches the station at 25 mph and does not stop. The starter signal is red, so the driver slows down. The brakes fail (for example). The overlap between the signal and the back of the train in front is too small and despite the tripcock putting the emergency brakes on, there isn't enough room to stop the train. BANG!

At 5mph, a brake failure (or any other SPAD circumstance) would easily allow the tripcock to stop the train before hitting the one in front.

Hope that helps explain these things to some degree ...

[editorsfoot]

Thanks for the comprehensive answer.

At least if something like this should happen then it wouldn't effect a challenger too much, as I can't think of a scenario when you would run to West Ken and then catch a train back to Earls Court, chances are you would go Earls Ct - Olympia - West Ken - Barons Ct and onwards.