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| Re: Scheduling Capacity (1353313) | |||
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Re: Scheduling Capacity |
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Posted by Stephen Bauman on Fri May 22 18:51:21 2015, in response to Scheduling Capacity, posted by BusRider on Fri May 22 17:09:43 2015. "Currently, 15 lines in the system cannot schedule any more service either due to having reached full track capacity, passenger capacity or both."Taking from an article on the MTA website regarding the new "Step Aside" signage on the floor of platforms. That's not true. It's an excuse used by the MTA to hide managerial decisions to keep down operational costs. The principal operational cost has been the elimination of split work shifts. (BTW, I'm in favor of eliminating split shifts as a safety measure.) However, it makes adding additional peak period trains more expensive than when more trains operated on the existing tracks with the same "obsolete" signal system. What determines the capacity? There are several components that determine capacity. Terminals are not usually a capacity determinant, except in NYC. This is due to bad design for newer reversing terminals like Jamaica Center and South Ferry. Reversing terminal capacity has been decreased because NYCT replaced faster acting electro-pneumatic switches with slower acting all electric ones. A reversing terminal's capacity is determined by the time it takes trains to traverse and clear the interlocking. The principal capacity component is intermediate capacity (between terminals). This is determined by the emergency braking distance. The idea is that the leader can stop in 0 time (derail or some other reason) and the follower must stop before colliding with the leader. This provision means the follower must maintain its emergency stopping distance behind the leader at all times regardless of the leader's speed. This means the follower must know in a timely manner its leader's position to a degree of precision and accuracy consistent with the operating tolerances of the emergency braking system. Intermediate stations add an additional wrinkle. Analytically, they are handled by assuming the leader is at the same distance ahead when the follower approaches the station. In order for that to happen the leader must have: slowed down to a stop from its station approach; spent dwell time within the station and accelerated ahead to a position that is greater than the follower's emergency braking distance. The principal parameters governing this are the service braking and acceleration rates, train length and the dwell time within the station. For most systems with standard braking and acceleration rates and a 30 second dwell time, the intermediate station capacity is a nominal 40 tph or 90 second headway. Uncertainty in locating the leader (block size or message delay for CBTC) increase this headway because this adds to the a longer safe distance between leader and follower. Longer trains increase the headway because the leader must travel further to reach the save distance from the follower. Headway should be the independent variable with dwell time being dependent on headway and not vice-versa. The shorter the period between trains, the fewer passengers there will be waiting to get on each train. When many lines share track. Merges are no different than interlockings leading to a reversing terminal. Let's assume merge interlocking is 300 feet long, trains are 600 feet long and trains travel at 10 mph over the interlocking. Trains must travel 900 feet to enter and clear the interlocking. If they travel at 10 mph, it will take them 60 seconds to clear the interlocking. If trains are scheduled every 120 seconds (30 tph), the interlocking would be idle for 60 seconds between trains. If trains were within 30 seconds of their schedule, then the leader being 30 seconds late and the follower 30 seconds early should not result in any delays at the merge point. Similarly, if trains were scheduled every 90 seconds (40 tph), trains would need to be within 15 seconds of schedule to guarantee no merging delays. |
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