The other night I saw film shot in the Bronx of one of the New Haven's electric box cab switchers working ground level track along the viaduct on the approach to the Hell Gate bridge. This was on the east side (using compass directions) of the viaduct, the side opposite where the tracks were that went to the Harlem River yard. I was surprised this track had been electrified but there it was.
This motor came out from between a building and the viaduct, moving south (away from Oak Point and towards the bridge) and then the crew flagged across the street. They were pulling a few cars, boxcars I think. The photographer was standing along the opposite curb shooting the movie. It was E.138th Street I think. There was no narration or captions but from the automobiles present it looked like the 1940s.
When I lived in Manhattan in the 1980s I visited the area a number of times. It's a tough neighborhood so I used to go on Sunday mornings when the bad guys were all home sleeping it off.
At that time the track was still there. Or at least the ROW. You could see how the track once ran along the base of the HG approach. I believe there were rails still embedded in the pavement. I don't recall seeing any evidence the track had once been electrified so I was very surprised to see the scene in the movie when the motor suddenly appeared.
At any rate, this was very urban, industrial type trackage. Obviously if the NH was using a motor to work this track the sidings would have had to been electrified as well. In addition to Noel Weaver's expert commentary above, I think I have read in articles and books by Bill Middleton, possibly Jerry Pinkepank and others too, that the New Haven once had quite an extensive network of electrified yard and siding tracks in the southeast Bronx as well as at other places.
Imagine if there had been an Internet in say 1940? Wowee!