I thought I had seen all the PRR electrics. I have never seen this before. It looked liked it could have been made for the New Haven, except this locomotive could have been paired with a twin. The back of the unit seems abbreviated. Any information/history on this unit and what line it could have served? The unit is an early example.
Michael Bezilla's "Electric Traction on the Pennsylvania Railroad 1895-1968" (Pennsylvania State Univ. Press, 1980) has an extensive description of this and the PRR's other early experiments.
But in sum: In the period between 1905 and 1907, the Pennsy was experimenting with electric locomotive designs and power distribution systems in order to decide what to apply for the Penn Station electrification. (They had already decided on 650-volt DC for the LIRR, but had not yet settled on the PRR itself.) Between 1905 and 1907 it built three experimental designs -- two DC in 1905 (#10001-2) and one AC in 1907 (10003, built by Baldwin-Westinghouse). The 1003's 2-B, or 4-4-0, wheel arrangement was intended to match the D16-class steam 4-4-0s, so that they could make direct comparisons in performance, rail wear, etc.
It was first tested against the two DC locos on a special stretch of the WJ&S's Camden-Atlantic City line, using a motor-generator set in the baggage car behind to convert the WJ&S's DC for the AC traction motors. It was then moved to the LIRR, where an 11,000-volt AC test installation was set up between Garden City and Babylon. As far as the locomotive was concerned, the results were good, but the electrical distribution system needed more development and the PRR didn't have the time. So the tried-and-true 650-volt DC system was adopted for Penn Station, using the DD-1 design as the standard locomotive power -- which you might say was an outgrowth of 10003's wheel arrangement. There seems to be no record of what happened to the 10003 afterward, but presumably it was scrapped after having served its purpose.