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| Re: MTA Bottleneck Report (1649959) | |||
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Re: MTA Bottleneck Report |
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Posted by Stephen Bauman on Thu Jul 30 09:08:55 2026, in response to Re: MTA Bottleneck Report, posted by zac on Thu Jul 30 07:26:32 2026. 1. Timetables need 1 second precision. The MTA's timetables have 30 second precision. They are effectively using an hourglass It takes approximately 1 minute for a train to pass through and clear an interlocking.Consider 30 tph (120 second headway) operation. Branch A trains arrive at: 0:00; 4:00; 8:00; etc. Branch B trains arrive at: 2:00; 6:00; 10:00. The merge becomes: 1:00(A); 3:00(B); 5:00(A);7:00(B); etc. Suppose the branch A train is 30 seconds late and the branch B train is 30 seconds early. The arrivals at the interlocking are: 0:30(A) and 1:30(B). The branch A train blocks the interlocking for 1 minute or until 1:30. It does not delay the branch B train. This means that trains must be within 30 seconds of scheduled arrivals to ensure no delays. Determining whether trains are within 30 seconds of their schedule is impossible, when the schedule precision is 30 seconds. The nominal service level capacity is 40 tph, assuming 30 seconds of dwell time at each station. This reduces the on schedule limit to 15 seconds. 2. In order to maintain uniform headways, merges must be balanced. This means both branches must have the same headway. Consider the Rogers Jct: southbound arrivals at Franklin Ave between 6:30 and 7:30 are: #2 @ 8 trains; #3 @ 5 trains; #4 @ 9 trains; #5 @ 9 trains. N.B. the 8 trains for the #2 could be a truncation error. However the #3 @ 5 trains isn't. Suppose all trains operated at 9 tph (400 sec headway). Consider the following schedule for Franklin Ave southbound arrivals. 0(4); 100(3); 200(5); 300(2) 400(4); 500(3); 600(5); 700(2); 800(4); 900(3); 1000(5) The Rogers Jct. merge conflict is between a #3 (New Lots) and a #5 (Flatbush). N.B. there's a 100 second cushion between each each #3 and #5. There's also a 100 second cushion between #2 and # 5 along Nostrand Ave. The downside is that the headways along Nostrand are not uniform. 3. The MTA replaced electro-pneumatic switches with electric ones. This significantly increased actuation time. This is a problem at reversing terminals because the interlocking must handle trains in both directions. This means 15 tph becomes 30 tph and 18 tph becomes 36 tph operation. See what item 1 implies for how trains must adhere to schedules to avoid switching conflicts. Actuation time must be added to the time required to clear the interlocking. This time is also compromised because incoming and outgoing trains are slower. Incoming because of the bumper block; outgoing because of the brake test. |
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