I would also agree on 503. Its seldom used, mostly was a backup for 566 and sometimes would see both if it was a large train or if they needed to switch PSNH power plant on their way south. (The siding faces north, tough to switch. I would guess it will go the way of 302 (the former RI GP-7) now with Hobo Railroad in Lincoln. 566 lately has been shoved in the Steinbecks siding along with 503. I saw 2370 twice yesterday switching Blue Seal in Bow.
Josh
Josh