The full Northeast Corridor electrification was completed and put into operation on February 10, 1935, which also brought the GG1 into full revenue service. The line from New York to Washington had begun electrification starting in 1905, both by the Pennsy and New Haven. I doubt K-4 Pacifics ran over the whole length before 1935 since sections, starting in New York, were progressively electrified through the 1910s and 20s. Both the Pennsy and New Haven operated electric locomotives over the completed sections rather than steam.
The only way to know for sure would be consists of Pennsy trains between NY and Washington, of which several were printed by Wayner over the years. I have some of his books, and can see what is shown if anyone is interested. One I have spans the 1920s through the 1940s.
