Lookuing North towards Assembly OL station.
IMG_3492 by
Bos Beeline, on Flickr
As explainer, this is the "runaround" switch for freight locos to change ends of their train. BO-1 is the Pan Am job that usually handles Everett and Peabody (sometimes there's a supplemental BO-2 if they're particularly busy), originating via the Lowell Line. The train will pull forward on one of these tracks (doesn't matter which one), then de-couple the leading locomotives. The locos will pass through this switch onto the single track, then reverse back over the switch onto the open second track and pass their set-off freight consist. That's called "running around the train". The length of single track from the switch to the Assembly bumper is long enough so a 3-loco lashup can comfortably pry off and reverse in tandem.
Most freight locos--but especially the ones assigned to local jobs--are double-ended and can run either in the cab-forward position like a passenger train or in reverse "nose forward" (a.k.a. "butt forward"). In reverse there's a rear-facing engineer control stand and rear windows on either side of the narrowed engine room enclosure for visibility...with 2nd crewmember stationed at the opposite window to help out with the primary engineer's slightly compromised binocular vision. Example from a Pan Am GP40. . .
Cab forward (normal running):
Same unit, "butt forward" (note rear-facing window on crew door and outdoor catwalk so 2nd crewmember can go outside for closer look and walkie-talkie w/engineer at other rear window)
(The T's GP40MC's, owing to their previous lives as freight power, were built exactly the same as this pictured PAR GP40-2 unit...only the rear control stands were deactivated when they were converted to passenger power in the mid-90's.) So after running around and passing their train the freed locos, running "butt-forward", will go about doing any other misc. business while they're in-town...often blocking or prying off some more cars from the other side of the freight car lash-up with shoving moves from the Charlestown end. And if they have to change facing directions to run cab-forward for the next job they'll make a trip around the wye behind BET (limited-visibility "butt forward" entails speed restrictions on mainline track and thus isn't great for running longish distances, so they will wye back to cab-forward at earliest possible convenience). When it's time to pick up their parked freight train and leave they'll back onto the tracks, couple at the
opposite (Charlestown) end that they came in, and pull the train forward to get out.
How Pan Am directionally manages Yard 21 setoffs depends on which legs of the round-trip (inbound/outbound 495-Somerville, inbound/outbound Everett/Peabody-Somerville...and combinations therein) they're using it most often as a parking spot. The single "Valley Track" wrap-around off the Lowell Line around the backside of BET--which will be disconnected/reconnected several times during GLX construction but ultimately retained when all is said and done--is long enough to temp-park a 90+ car freight car consist between passenger congestion. So if PAR is running on-time they should be able to scoot back out to 495-land grabbing the next opportune slot without needing to can their freight train overnight at Yard 21 or park it 6+ hours between crew shifts (where they'd need to get off the Valley Track so T/Amtrak have enough maneuvering room behind BET). The problem is that PAR never bloody runs on-time because "slop ops" is a way of life for their adorably half-assed corporate culture, so on current schedule margins (or lackthereof) they're going to need Yard 21 a real lot now that GLX has torn out all the Yard 8 tracks that used to split off the Valley Track on the west side of BET.
What's surprising is that I don't see any hand-throw crossovers going in at midpoint intervals on Yard 21. If they were doing switching activity (breaking apart/combining strings of freight cars onsite in addition to just parking) like they did in Yard 8 the long linear double-track with single runaround switch at Assembly isn't nearly enough capacity for them to work with. One track would have to be kept clear 100% of the time for running around the locos, which hamstrings their storage capacity bigtime. Breaking up the track length at third- or quarter-points with crossovers would give them multiple points for running around or accessing freight cars for switching, and allow them to chunk out portions of the 2nd track for straight-up storage instead of having to keep it always clear. I'm guessing (because it doesn't take a lot of guesswork with PAR) that they just forgot to put in the request for midpoint switches when the T gave them the up/down vote on the track schematic and they're just going to pull a giant "Oops!" when they start using it and find out the capacity of this layout is way lower than what they need. No biggie...they can splice in a switch on their own time for a few grand in one afternoon. But nothing would be more quintessentially Pan Am than totally spazzing on this consideration until
after all the freebie work they were gifted was already finished.
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