|
|
| Re: LIRR East Side Access / Roosevelt Island F Station (557332) | |||
 |
|||
| Home > SubChat | |||
|
[ Read Responses | Post a New Response | Return to the Index ] |
|
||
|
Re: LIRR East Side Access / Roosevelt Island F Station |
|
|
Posted by Stephen Bauman on Mon Jan 21 17:31:07 2008, in response to Re: LIRR East Side Access / Roosevelt Island F Station, posted by Russ on Mon Jan 21 14:45:14 2008. Yes, running the SAS onto the QB local tracks is one more merge, but that's life....If extra merges prove to be a minor issue,That extra merge is not a trivial matter. The TA's routing of multiple merges/diverges from terminal-to-terminal leads to scheduled merging delays. Here's why. The simple system is two single services depart from terminals A1 and A2 merge at point B. They then proceed together along the trunk line to point C where they diverge as single services to terminals D1 and D2. The departures from A1 and A2 can be scheduled so that not only will there be no merging conflict when the trains arrive at merge point B but the joint service along the trunk will have uniform headways. Such scheduling is possible at any service level up to the capacity of the joint service along the trunk from B to C. Consider the case with multiple merges/diverges. Two services again start at terminals A1 and A2, merge at point B and diverge at point C. However both services merge again at point E, with one service passing by point D1 while the other service pass by point D2. The two services continue along a trunk until point F, where they permanently diverge into single services. Again departures from A1 and A2 can be scheduled to avoid any merging conflicts at point B and uniform headways along the trunk between B and C. What about the merge at point E? The problem is that the travel times between C and E differ depending on the routing via points D1 and D2. There will be no merging problem, if the individual service headways and the difference in travel times are an odd multiple of the other. However, service headways are adjusted to suit demand but travel time differences are set in concrete. Thus, if either the headway or travel time difference is an even multiple of the other, then a merging conflict at point E is guaranteed. The TA is guilty of many such multiple merge routes. Most are slightly more complicated, in that the same two services don't do multiple merges. There are still multiple merges but second merge is with a service that was linked to one of the services at the first merge. There are two examples of a double merge with the same services, that come to mind. During the morning rush hour on the White Plains Road Line, 2 and 5 trains merge at 238th. They then diverge at E 180th, with the 5 running express while the 2 runs local. They then merge again at 3rd Ave. Another example is the northbound B and D during the morning rush hour. The B and D merge at 36th St in Brooklyn, diverge at 59th-Columbus Circle and merge again at 145th St. It's not always the tower operators. Sometimes the schedule makers are to blame. |
|
|
|