[personal profile] dmaze
(0) They removed the signal behind the Market Basket, and changed the area between Sherman St. and Alewife Brook Parkway to no longer be a signaled junction, which affects what the signal at Porter can show.

(1) Old signals showed red, unless the track was running in this direction.  New signals show a normal block signal aspect unless the track is running in the opposite direction.  (Or else, new control system makes it harder to set default "trains run on the right" routes and forget about it; but I'm pretty sure I've seen the old signals show red in both directions before.)

(2) The new signals take quite a while to reset to red when a train passes, like 10 seconds or so.  (It is possible that the train needs to completely pass the signal, but I don't think it actually works that way from watching two trains go by, and it would be odd for a new installation at this location anyways.)

This in combination means that the new default is yellow-over-red (approach) on both tracks; red-over-red (stop), probably on the left-hand track, means a train is coming from North Station; yellow-over-green (approach medium), probably on the right-hand track, means a train is going to North Station; and green-over-red (or -over-green, clear), for wonderfully geeky reasons, means a train has a route set all the way into a platform at North Station.  The outbound signals only show red/yellow/green; I have not yet observed whether these (or other new installations, like on the Lowell line) show flashing-yellow (advance approach) (rumor from the Internet is that they don't).

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