All this talk of "UL" listings aside...getting back to the original question.
I have been running my "electronic" - equipped locos with an MRC 027 "Dual Power" unit; much newer than "old style lionel transformers". It runs them just fine.
I did try an American Flyer 15B with the same steam engines, just to see how they would perform. The speed control was not as precise, but they did put out more smoke...I'm guessing the 15B's minimum voltage is higher than that of the MRC unit, so the smoke resistor heats up quicker(?)
Mark in Oregon