This is slightly simplified, but a traditional signalling system essentially works on the stopping distances of the trains, which are a lot longer than the stopping distances for a car, because the steel wheels have a lot less grip than a rubber tyre.The track is divided into blocks, with electrically insulated joints between them. When a train enters into a block, it completes a circuit between the two rails via the axle. It then turns the signals behind it red.The length of the blocks is the key thing. You have to leave enough distance ahead for the train to stop safely. When the signalling is designed for high speed, those blocks will be longer. If a train goes through at low speed, it will take much longer to clear the block, and it will hold the signals behind it red for much longer, and potentially hold up the train behind.You could get around it by having shorter signalling blocks, even in higher speed track, but then you have more insulated joints, which are a point of weakness in the rail, and you will get more signal failures, and more cracked rails.The signalling around Turnham Green was designed for high speed trains. If you stop trains, they take too long to clear the signalling blocks, and you are holding the signals behind at red for too long, so you delay every single train. You then increase the gaps between the trains. Those gaps will then apply on the whole of the line, and you reduce capacity. It can stop early in the morning and late at night as the train frequency is much lower, so you don't hold up the train behind.Modern signalling systems with automated trains use different methods of locating the trains. They also know the train's speed, which allows the trains to get closer together. The current signalling system on the Piccadilly line is dumb, so it has to work on the principle that any train inside a block is stationary. If you know that it is moving, then you can get the train behind closer by the stopping distance of the train. The lines with more modern signalling can get somewhere around 33 or 36 trains per hour. With conventional signalling, they are around 24 trains per hour. From memory, the Piccadilly line went to 27 or 28 trains per hour in the early 2000s, but reverted back. At the higher frequency, trains started backing up at red signals as soon as there was the slightest delay.
Al Webber ● 2164d