All of the EP-series electrics were physically capable of running under Pennsy wire. It was all 25-cycle, 11,000-volt current. Coming off the New York Connecting into Penn. Station it was Pennsy wire. The EP-1s and EP-2s didn't run into Penn. Station, nor were they allowed to, since they lacked cab signals. The EP-3s, EP-4s & EP-5s, however, did so on a regular basis, as did the passenger-equipped EF-3s.
The only New Haven electrics to run west of Penn. Station were for test purposes; the EP-3s to help develop the GG-1, and one (maybe two) EP-5 test runs (for possible run-through service). The EP-5s were well-suited for the New Haven, but not the PRR; their traction motors developed serious overheating problems due to the long (by New Haven standards), high-speed running to Washington, D.C. One run to Washington in the morning and they'd have to spend all day cooling off so that they could make the return run. The GG-1, on the other hand, could arrive in Washington, get a quick servicing at Ivy City Engine Terminal (oil, sand, water) and make a return run to New York with minimal delay.
Following the merger of the New Haven into Penn Central, the New Haven's EF-4s, now classed as E-33s, ran out their last miles on former PRR trackage. Although not as fast as the PRR's E-44s, they impressed the former PRR people with their pulling power.
Ex-NYNH&H SS Opr