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Re: TA is obsessed with CBTC, and ''New'' tech for no reason.

Posted by trainsarefun on Sat Mar 1 13:24:38 2008, in response to Re: TA is obsessed with CBTC, and ''New'' tech for no reason., posted by Stephen Bauman on Sat Mar 1 12:45:53 2008.

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There are two things you want to measure. The first is the acceleration of a train leaving a station. This is the time interval from when a train starts to move until the signal at the entrance for an incoming train turns green. The second is the braking rate. This is the time interval from when a train passes that green signal at the station entrance until it opens its doors. If the sum of both these time intervals is less than 60 seconds, then 90 second headways (40 tph) with 30 second dwell times are possible....

These measurements are taken during off peak hours...



I suppose that I might be comparing apples to oranges, in a way, then since I'm probably referring to peak or quasi-peak pressure, the sort of thing that happens when, say, you have three trains coming through 60th St Tunnel headed southbound. The R train becomes the leader by crossing over ahead and proceeding through the tunnel first. An N train from Astoria follows quickly behind it, and behind that N train comes the second following train, working the W route.

The leading R train throws up station time signals behind it, so the N train is closing on Lexington Av station at reduced speed. It isn't (always - depending on the number of signals in the station itself) the case that the first trailing N train will enter Lexington Av station to further close in behind the departing R train, even if the entering signal displays green, because the trailing N train will wait for the rear of the R train to clear the station before the N train enters the station. In some cases, the entering signal will be green for quite a few seconds while the rear of the leading train clears the departing signal. Effectively, this would stand, I think, to lower the acceleration rate term. (The second trailing W train will act behind the N train as did the N train behind the R train).

Now, I suppose the easy answer is to say: 'Well, don't run the trains like that - that's a timetable issue, not a signal system issue. The signals are supposed to deal with regular headways, not one train leading two others trailing right behind it that aren't going to use the station time signals, either.


(trailer)--G-.........Y..........R.......R...........R.---(leader)

moving that way ------>

---- = tunnel

..... = station

(Y) or (R) = signal that only displays yellow/red aspects

So in this very crude schematic, the entering signal displays green, the following signal gives yellow, followed by reds. The trailer won't advance at restricted speed to close in on the nearest red signal, it will remain at the entering signal until, roughly, since this is me keyboard-drawing:

(trailer)--G-.........G..........(Y).......Y........R.----(leader)

Only when this latter state occurs will the engineer will then accelerate his train and at some point begin braking to platform, and the cycle goes on. (All signals except the rightmost one are on station time speed).

One item on my to-do list is to get out to Canarsie and measure the terminal's capacity.

Not balmy enough for you, yet? :)

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