I don't know specifically how the caboose came to rest here at the end of the Dorchester and Delaware, but I do know that the company that now owns the adjacent depot was founded by a guy who was originally from central PA, and whose family had a major coal contract with the PRR back in the day. The inside of the depot is festooned with much PRR finery, including the original coal furnace complete with nickel plated fittings and trim.
At one time the caboose was used as a snack bar. It would be nice to see it fixed up. For a time, Nabisco had a plant in Cambridge where they made Grey Poupon (But Of Course!). They received mustard seed or something in big Soo Line covered hoppers with the wheat logo. That ended about the same time their employment tax credit did...
Lee