Happy to be corrected by someone that knows better than me, but you might be understating the significance?
It looks like it's been their staging location/access point for track maintenance lately.
It is a fairly long storage track right next to the portal to Kenmore and the central subway. It looks like they can get large trucks and heavy equipment in down there and transfer materials between maintenance train + storage area/vehicles. It's also mostly out of the way from the view of the public and has relatively limited residential abutters, which probably keeps pressure off about keeping what look to be big piles of ballast, ties, and tracks over there in recent years.
If I take a look at the
Street View off the bridge, the 3 most recent images (2017, 2018, 2020) look like it's in active use for maintenance purposes, and the most recent overhead imagery shows something parked on the tracks.
I'm also not sure if there's any other convenient location for this sort of thing other than all the way out at Riverside, and if so, I'm not sure how detrimental it is to efficiency of maintenance to push it all the way out there.....but it seems like it could be fairly significant?