| Topic: | Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Mike Brown's Longing for a Piccadilly Line Stop at Turnham Green | |
| Posted by: | Al Webber | |
| Date/Time: | 13/02/20 10:59:00 |
| Francis, the way the signalling works is like stopping distances for a car. You need to leave a decent distance between you and the car in front. If you have driven on motorways, you have probably seen those chevrons and signs saying to leave 2 chevrons between you and the car in front. 2 chevrons is less than the stopping distance at 70mph, but is fine when the other vehicles are moving at similar speeds, because they take time to slow down as well. The problem with older signalling systems is that they sort of know the location of the train, but they don't know the speed. Because of that they have work on the case that it may be stopped and leave the full stopping distance - not just the railway equivalent of 2 chevrons. It's made worse because all they know is that the train is in a particular block of track. They don't know where in that block the train is. For safety reasons, the signalling system has to assume that the train is at the beginning of the block. Also,stopping distance for trains are a lot bigger than for cars. Steel wheels on rails do not have nearly as much grip as rubber tyres on roads. You could get around this by having more and shorter signalling blocks, but that gets a lot more expensive, and will also lead to more signal failures. The signalling system is designed around parameters of train speeds and the planned intervals between them. Newer signalling systems know the location of the train far more accurately, and also know it's speed, so you can get trains far closer together. When you get to the specifics of Turnham Green on the Piccadilly line, the signalling blocks on the Acton Town to Hammersmith stretch are very long - because they are designed for higher train speeds. If the trains slow down to stop at Turnham Green, it will take them much longer to clear the signalling blocks, and you won't be able to maintain the gap between trains. If you look at the working timetable that Michael Robinson linked earlier, from 06:50 there are trains every 2 and a half to three minutes. That is about as close as you can get trains together on any line with conventional signalling. It would be impossible to maintain those frequencies with a stop at Turnham Green with the longer signalling blocks there. |