Lionel Pennsy S-2

[Charlie B]

Ed, that is an AC powered unit. you can tell by the "E" unit in the center, which reverses the direction of the locomotive . They would run on DC too, the whistles in the tender were DC operated, when you turned the whistle switch at the transformer it put DC to the track and the whistle fan would only run on the DC power. you would also notice a slight increase in speed from the locomotive motor getting an extra shot of power from the DC.
I'm sure one of our "O" gauger's will provide a better explanation.