It's called the Grand Junction. CSX used to use it to get to the produce market and scrap company on Everett. That business has all gone to Pan Am now as CSX doesn't come east of Framingham anymore.
Pan Am never used it as far as I know.
Amtrak also uses it on occasion and one of the most interesting moves was taking the "Great Dome" "Ocean View" on it to get from the west to run on the Downeaster. This is as close to Boston South Station as a full-length dome has ever been. A regular dome has been in South Station at least once (Quad Graphics private car - came in via the Midland) and Amtrak did run an experimental dome car test to see the possibility of putting a dome on the LSL a long time ago. They felt the clearances were too tight unfortunately. That would have been a great asset on the route.
Another interesting tidbit is that when the Ringling Brothers Barnum and Bailey Circus train came to town, the passenger cars were parked on the double track between Mass. Ave. and Memorial Drive (gaps were left for the pedestrian crossings).