The Paint Track is a vestige of the old 3rd Iron (I believe) that went all the
way out to W.Cambridge (along with the 4th Iron on the west side of the
mainline). But that was long ago.
In more modern times (Tower H), it was known as the Paint Shop lead and ended at Prospect Iron. It was also how the B&M switched the A&P
warehouse (now Brickbottom) and where the PC/CR set off its B&M
interchange traffic.
With the reconfiguration Tower H and replacement by Swift Interlocking,
it became the Paint Track, rebuilt to 1800ft in length. Its used for shifting
moves, work train equipment storage. The Northside Work Train was
likely doing either or something related. The cars on the western end
by Target are not in service and includes an ex-PRR hopper (which
vandals started a debris fire in this past Fall!
Dave