Ron Newman wrote:Also, I've heard that some Fitchburg Line passenger trains were occasionally diverted onto this branch in the late 1970s or very early 80s -- possibly because of flooding or construction.
Yes, that's true, and it was because of flooding. There was a low spot on the Fitchburg line that was prone to flooding in heavy rain, located where it passed under the now-removed multi-track freight bridge that was adjacent to old Tower H, the Brickbottom art studios, and McGrath Highway. Because the freight cutoff diverged from the Lowell line and avoided that spot, Fitchburg passenger trains could use it as an emergency alternative and rejoin their regular route in West Cambridge.
I believe that the Fitchburg line track in that area was raised up a few years ago when the freight bridge was removed following Guilford's yard closure.