Cab signals?

Discussion relating to commuter rail, light rail, and subway operations of the MBTA.

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Re: Cab signals?

Postby F-line to Dudley via Park » Fri Mar 27, 2015 1:27 am

mvb119 wrote:
dbperry wrote:fascinating stuff. Another question: all things being equal, does having cab signals increase the allowable density on a given line? Or does it not matter, because the blocks are still the same size and dictated by the speed limits on the line?

Doesn't really matter much since the blocks are the same, however it does help to speed things up. When a train passes a wayside signal showing anything less favorable than a clear signal, it must proceed according to that indication all the way to the next signal, even when the train in the block (or 2 ahead) clears that block. With cab signals, the train has a constant stream of code through the rails. The train will instantaneously know when the block or blocks ahead clear since the cab signal will upgrade right away no matter where in the block the train is so the engineer will not have to wait until the next wayside signal to increase speed. The only exception is having a restricting in the cab. You can't increase speed right away, since the rulebook requires you to not increase speed until the train has traveled 500 feet or 1 train length (whichever is greater) past the point where you received a more favorable cab signal indication. This is done mainly in case the cab signal flipped in error. (for all you know the rail ahead could be broken causing the cabs to drop out, but it momentarily makes contact with the other rail again causing it to go in and back out again) Restricting is the fail safe of the cab signal system since it relies on no code being in the rails, so the train must travel at the slowest speed, restricted speed.

And this is also the pickle in upgrading lines for cab signals. When you have continuous track circuits on a wayside-only line, you can add the cab signal layer. Because that's all it is: an extra layer on top of what's already there. The track circuits can already sense the train, so what you are adding with cab signals is the 2-way communication of train sensing the track circuits. That spanking new Fitchburg Line signal system...not real costly to add on later, and doesn't "waste" any of the work they did. But you can't modify like that with some of the telephone-line fed ABS that's still littering the more ancient northside installations or the inner Worcester Line where there either aren't track circuits at all or they aren't continuous track circuits. Therefore, the Reading Line is going to be a total do-over if they ever get Pan Am permission for a cab signal installation there because there's no communication through the running rails the way that ancient wayside system works. Whereas (in theory, at least) it shouldn't be a backbreaker to do a non-invasive modification on the Needham Line (caveat: but sometimes scrapping older-generation copper cable for newer and more reliable fiber optic cable just is that much worth it for the one-time replacement pain, so "non-invasive mod" isn't necessarily what they'd choose or what would be recommended if given the choice).
F-line to Dudley via Park
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