What's up at Needham Heights?

[dmaze]

Here we're in "technical train question" territory, complete with ASCII art. Yesterday I was bumming around the Needham Heights commuter rail station. It's the end of the (largely single-track) Needham commuter rail line; that line runs north-and-south here, but northbound, the direction more towards Boston ("physical-inbound"), is the opposite direction from the way commuter trains to Boston actually go ("logical-inbound"). I suspect there's also some freight traffic, continuing physical-inbound across 128 into outer Newton industrial land. It looks like this:

-- To yard ----------\   (3)                 (4)(5)
-- To Boston ---------\------------------------------ To Newton ---
                 (1) (2) *** Platform ***    (4)

(1) is a three-head color-light signal, except it only has lights to show "stop" (red-red-red), "approach" (yellow-red-red), or "restricting" (red-red-yellow). (2) is a hand-thrown switch. (3) is what I'll call the "magic signal box"; it'd be on the right-hand side of a logical-inbound train at cab height. (4) are signs saying "END ABS" for logical-outbound trains. (5) is a single-head color-light signal with only red and yellow lights.

My GIMP foo is not good enough to get a non-garish picture of the magic signal box, but see here and here. It has, in four separate columns, position-light aspects for "clear", "approach medium", "approach", and "restricting", plus a red button.

If I was going to guess at this, I'd guess there would be an interlocking (0) off the left side of above. Logical-outbound trains would get a yellow signal there, taking them through the "END ABS" sign (4). Freight trains proceeding further (the right-of-way does look to be maintained) would have special clearance. Logical-inbound trains would get a red or yellow signal at (5) depending on whether the platform block is occupied or not, and the signal at (1) is either "enter yard" or "proceed on main line" depending on the switch (2). The "BEGIN ABS" sign is probably at (0). The box at (3) can be used by commuter train operators; if they press the button, the box lights up, showing "restricting" if the switch is set for the yard and otherwise repeating the signal aspect at (0). But this is all speculation.