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| Re: SAS termini questions (127287) | |||
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Re: SAS termini questions |
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Posted by Stephen Bauman on Mon Aug 15 15:04:56 2005, in response to Re: SAS termini questions, posted by stephenk on Mon Aug 15 13:45:56 2005. At 2 minute headways it is not realistic to schedule trains to arrive excatly on time for their parth through the crossover, as any train running late (which is quite common on metros!) will delay the departure of the next train out. So it makes sense to schedule the train to arrive at the crossover a little bit early, and wait for its path through the crossover.There is a window by which trains will not encounter any delays. I also showed how to calculate this window, based on the time it takes to traverse the crossover and the headway. It makes most sense to make sure that all trains operate within this window. To the make the most of this, the signal protecting the crossover should be as close as possible to the crossover. This tends to require speed control signals, or ATO. There's a safety concern here. The signal system must be able to stop a train before it hits the crossover. This distance is essentially the same for a block system and systems with speed controls or ATO. The signal system was also designed for degraded operation - the possibility that the brakes might be disabled from the lead car. The IND designers never thought that their successors would be so stupid as to introduce operating equipment were a single brake system would control more than one car. Two passengers were killed at Roosevelt Ave because of this. Even with high speed crossovers and long overruns, the realistic capacity at 2 track termini with scissors crossovers is still circa 30tph. High speed crossovers require more space, due to the larger curve radii, which limits the advantages of having the high speed crossover in the first place as the train has to travel further to clear the crossover. The Times Sq. station on the Flushing Line operated at 36 tph for a couple of decades according to the TA's own published data. Try to get some perspective on how much a factor the switch is. Let's assume it is 200 feet long (including signal protction) and that we are dealing with 600 foot long trains. This means that a train has to traverse 800 feet to enter and leave the switch. If this train takes 60 seconds that means an average speed of 9 mph. Let's assume that the average speed is to be increased to 15 mph - hardly a "high speed" crossover. This reduces the 60 second traversal time to 36 seconds. How much margin did those Flushing trains have? They operated at 100 second intervals and spent 72 seconds in the crossover. That gave them a margin of 28 seconds. So each train was expected to be within 14 seconds of its schedule. That is reasonable. That is what Moscow does. |
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