Recent reports on the NERAIL discussion list indicate the onetime Boston & Albany roundhouse at Athol, Mass. was razed a couple of years ago. If I recall (having never actually seen it), it was a very old (probably Civil War era) two stall structure, with arched doors. Perhaps another reader can provide more details and/or a photo.

http://www.rypn.org/Database/NERoundhouse/
ryarger@rypn.org