Roughly: The restored Yard 21 replaces other freight train-parking space lost, particularly where the GLX has already squeezed freights out in this same area (but more on the E. Somerville / USq side)
It is equivalent-capacity car storage space to what was lost when Yard 8 on the west side due south of the Washington St. overpass closed last year to get ripped out for GLX. But slightly less flexible for switching cars because of the way Yard 21 is pinned in by the Orange Line (though if Pan Am gave two shits about efficiency they could effortlessly self-pay for a couple more hand-throw crossover installations on Yard 21 to make switching ops more nimble than the default T-paid setup). PAR were the ones who sold themselves short by cashing out their real estate rights to Yard 8 in the GLX land swaps, so it's their nickels-on-dollar to spend if they need to make Yard 21 ops act a little more smoothly than the barebones current switch setup.
Yard 21 is set up in a "runaround" track configuration...single-track at the FX Interlocking and Assembly ends-of-track, double-track in the middle. This is so they can detach the locomotive from a parked string of cars and "run around" alongside it on the open second track to change ends for the prevailing operating direction. So even when the car counts aren't long enough to stretch over to the Sullivan platform you will see the PAR locomotive putter past all the way to Assembly for the sake of changing tracks/changing directions.
As described previously, Pan Am is getting shoved here because it's one of the last nooks in Somerville they
haven't land-swapped to the T for sweet instant-gratification cash. They still legally have perpetual squatter's rights on this spur, whereas those perpetual rights at Yard 8 and Northpoint were voluntarily bartered away in the land swaps for $$$. It's nobody's idea of everlasting permanent digs, and if they get acquired by a carrier who gives a damn about running on-time they won't need Somerville nearly as often as a storage crutch for running out of crew hours. BO-1 on its Peabody days is a prime example of how stoopid their PAR 'slop-ops' go. To switch Rousselot Gelatin's small siding, PAR has to sometimes make as many as 4-6 switching trips over the 2-1/4 miles between North St. Yard outside Salem Station and Rousselot's siding on the South Peabody Branch to shunt 2 cars at a time. Requiring a crossing of dead-center heart of Peabody Sq. every time...throughout the duration of PM rush...with the crew needing to hand-flag all crossings because they're too cheap to install flashers and/or gates on their own track through the Square or pay in to the T for crossing protection upgrades to the 4 crossings immediately prior to the Square. What's more...their refusal to patch one 20+ year old track washout on the out-of-service South Peabody Branch track just west of Rousselot means they can't/won't use 3200 ft. of perfectly fungible self-owned track for extra storage space that would basically eliminate all those inane fetch moves through the Square back to Salem. Therefore, the understaffed BO-1 crew habitually runs out of work hours before they finish switching Rousselot and has to dump their return train in Salem to come back and finish the next day. Which in turn puts stress on the next day's understaffed BO-1 from finishing its job within hours. PAR is rumored systemwide to light upwards of $1M per year on fire strictly for taxicab fare bringing crews to/from remote trains that ran out of Hours of Service.
Since 99.9999% of prospective railroaders give way more shits than this on the cost/benefits of running on-time, Yard 21 and other parking spots will probably be seldom-used by the next regime that replaces PAR. Throwing 10 grand into fixing the South Peabody washout and maybe finagling another $75K on top of that to install a double-track runaround between the Allens Ln. and Summit St. crossings is enough to perma-end all the back-and-forth lollygagging across Peabody Sq. and get them back to Somerville 2 hours sooner. They'll literally pay back the chintzy up-front capital investment in taxi fare saved. This in turn makes the Everett-only days on BO-1 run faster, and lets them get back home to Lowell or Lawrence without
any Somerville overnight parking. Staffing beyond the legal minimum also means switching gets done way faster and they can pack more customers on-trip. A one-stop shopping destination like Everett Terminal shouldn't take half a shift to switch by its lonesome, but for the fact PAR refuses to pay more than 2 guys to do it.
So in the future when the T needs to go to the land-swap well again to pick up Yard 21 for the Urban Ring ROW...the loss of Somerville's last overnight parking spot won't crimp freight ops. The new carrier will by give-a-damn purposes be running with extremely little Somerville parking (save for the short-term pauses behind BET between opportune commuter slots). And they can be compensated for growth + contingencies by the state with some grant money for expanding storage inside Everett Terminal and on the Winchester-Woburn stretch by reanimating 90% derelict Montvale Yard to bookend Somerville with enough safety-margin parking spots to safely give up Yard 21. The only reason why a Montvale + Everett triage isn't worth spending on today is that PAR would just make itself that much lazier...bogarting the new space as immediate parking lots while staying needy as ever in Somerville and setting new records for pure-waste annual taxicab fare for crews that can't stay within Hours of Service.
Right now--and under the current PAR administration--GLX is a legit worrisome complication for making fluid freight ops in Somerville a little bit harder. As Yard 21 is just a little bit ham-fisted a replacement for lost Yard 8. But that's largely a self-correcting glitch. Get rid of the FAILroad HQ'd in Billerica for an honest-to-god professional industry outfit and we're largely set for life...including when things like the Urban Ring ROW send next steps into motion.