Vest pocket locomotive
Template(s) drawn by Joshua Moldover, via The Railroad Paint Shop, and by myself, via my dining room table
Originally SLR 1000, this extraordinarily tiny boxcab is part of a class of locos tiny enough to fit onto Boston Elevated trackage (The SLR 1000s were designed to fit the Boston Elevated guage because both the Boston Elevated and the SLR thought that through running of parcel freight might happen, but it never did and the only time a 1000 ever ventured onto the elevated was to deliver mu cars.)
When the PV&T coverted the SLR to 3kvdc, most of the 1000s went to scrap, but the class engine instead went to the Portland shops and was rewired for 3kvdc, emerging like you see it here without 3rd rail shoes or a trolley pole, but with pilots attached to the trucks, corner steps, an all weather driver’s window, a pan (on a platform to reach uuuuuup to the PV&T height overhead), new roof-mounted lights, bell, and horns, and finally a spiffy blue & white paint scheme.
It’s /considerably/ less useful than the 320s that replaced all the other third rail units, but the tininess means it can work its way into sidings that a full-sized locomotive cannot.