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| Re: SAS termini questions (127084) | |||
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Re: SAS termini questions |
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Posted by Stephen Bauman on Mon Aug 15 05:51:11 2005, in response to Re: SAS termini questions, posted by Rail Blue on Mon Aug 15 04:56:23 2005. flat crossovers at termini are often a nuisance. I've lost count of the times my District's been held going into Wimbledon because something was leaving platform 4, only then to arrive and find the other three platforms empty.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. |
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