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| Re: SAS termini questions (127260) | |||
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Re: SAS termini questions |
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Posted by stephenk on Mon Aug 15 13:45:56 2005, in response to Re: SAS termini questions, posted by Stephen Bauman on Mon Aug 15 05:51:11 2005. "The "secret" to operating flat crossovers at terminals, or any merge point for that matter, is to adhere strictly to a schedule that does not have conflicts. How strictly? Suppose it takes 1 minute for a train to enter and clear a switch. If trains are operating at 3 minute headways, the switch will be occupied for 2 minutes out of every 3 minutes. The 1 minute that the switch is not occupied is the recovery time for late/early trains. Divide this time between the entering and leaving train, and a sufficient condition to eliminate switching conflicts is that trains must be within 30 seconds of their schedule. Want to raise service levels to 30 tph (2 minute headways)? It will still take 1 minute to clear the switch but the switch will be occupied for 2 minutes out of every 2 minutes between incoming trains. Oops, there is not recovery time. The trains will have to be operating within one second to guarantee no switching conflict.Solution. Cut the time that it takes for a train to pass over the switch. Overrun tracks at the end of the terminal mean that the train can operate into the station at near normal speeds. Also, switches can be designed to permit greater than 15 mph operation. Suppose this reulted in reducing the time over the switch to 45 seconds. Then 30 tph operation would require that trains be within 15 seconds to avoid switching conflicts at the terminal." 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. 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. 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. |
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