That's the yard at Fitchburg Sec. end-of-track, which does miscellaneous set-offs for couple customers who side-load to truck. Probably not that siding because the Hannaford Supermarket and Northgate Plaza Shopping Center took up residence in the adjacent former factory. The choicest rail-accessible parcels are just south of the 117 overpass where you literally can't spit without hitting a derelict siding switch.
Possibly it's the dirt parcel of a brand new building going up on Tanzio Rd. because Google shows a couple assembled track panels for a brand new switch sitting next to the ROW at that property. Last year's new signee is on Fuller St. (Google showing 4 cars spotted on their loading dock); prior to them coming the yard set-offs were the only biz left on the final 6 miles of track past Bestway Lumber in Sterling south of 190 Exit 6. It's a mystery why CSX Marketing suddenly reversed itself into give-a-shit mode way up here after 20 years of total indifference, but it's welcome to see given that the land use and highway proximity is pitch-perfect for rail-accessible light industry.