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I was reading earlier that all services out of Paddington are essentially stuffed due to signalling cabling being stolen. It seems that the cable was stolen in an area which effects the signalling in and out of the station. This is very bad news and means that despite there being four platforms there only one train can be there at any time. And herein follows a very quick and dirty explanation of how signalling works. I may do more of these later if they prove popular. Perhaps a series. With spin-off merchandise. Please note there will be a lot of mostlies and generalies because it’s not actually as simple as I’m going to pretend and I’m only going to really cover the most basic type of signalling.

Essentially, signalling is an on-off system. The track is divided into sections which may be occupied by only one train at a time. When a train is sitting in a section the signal behind it turns red so that no other train may enter until it has fully left the section. It works by passing a small electrical current up the running rails (the ones the wheels go on). If the current can get right up it’s rail to the end of the signal section then there’s no problem and the signal can stay green. However…

Should something large and metallic happen along such as – say – the wheels of a train then the current will flow across the axle instead. This is a very basic short-circuiting of the system and the signal for that section turns red. Pretty much everything you can think of on railways has come about due to some disaster or other and over time the failsafe option has been found to work better. So when something fails it does so in the safest possible manner. So every time a train enters a signal section the current is short-circuited and stops going to where it was supposed to go. And I’ll just add at this point, this is not the same powerfeed as that which supplies the signal itself. If there is a train or a short-circuit for some other reason then obviously the signal needs to show a red aspect (though actually, if there *does* happen to be a power cut to the signal itself and it has no aspect then we treat it as a red). The cabling which has been stolen would appear to be the feed for the track circuit.

When the train leaves the signal section then the signal can return to green (usually) again as it is safe for another train to proceed past that point.

So we have an on-off system. Train in section = red light on. Train not in section = red light off. Very simple (sort of).

What’s also neat about this system is that you can work out exactly where each of your trains are by gathering data on what the signals are doing. If Section 1 shows as being red then you know there’s a train there. If Section 2 then turns red you know that the train has moved forward. Both sections will remain red until the train has fully left Section 1 which will then go green again. This is the basic way in which The Voices know where we all are. Of course, it’s all very well knowing that you have thirty trains whizzing about but if you don’t know which is which then you’ll have issues. So another system comes into play which (usually) identifies the number of the train.

The issue facing the signallers for Paddington Station is that a terminus is a place with many, many signals. There are all sorts of signals which serve a variety of purposes. You know those guys at the airport who wave paddles around while walking backwards before a plane in order to instruct the pilot to go this way, that way, slow down, straighten up a bit…? Well signals can be set up to do the same thing (aside from the straightening up bit which I now realise was a stupid thing to write). So – LOTS of signals going on. And because of the interruption to the electrical flow to the signals they’ve all turned red (sort of). And because they are all red then the signallers have absolutely no idea which section has a train in it and which doesn’t. And there’s the additional problem that an entirely separate failsafe means that points will not move during all this uncertainty because the typical sign that a train is present is presented and points will not throw at that time as nobody wants a derailment.

It’s all very tricky. What’s most likely happening is that one platform has been selected as the working one. And staff have gone down and manually secured every set of points from that platform to the end of the problem area. So there’s one clear route where a train can be moved in and out. But only one train. Because if a second train happened to be moved into the area then it could conceivably run into the back of the first one and nobody would have noticed until it was too late because the means of tracking exactly where the trains are has disappeared. One train moves in then is taken out and clear of the whole mess before the next one is permitted in.

Signalling – simples innit?

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