Matt Doti said:
The "doughnut of death" is an inductor. At least that would be my guess. It sure looks like one.So in practice I can see why it reduces top speeds and starting speeds. The LGB mogul have this too as my newer undecorated model would not move until 5 volts or so. I believe this is so the lights and smoke get enough juice to work right before the loco starts moving. My old mogul (2019s) would move at the slightest voltage increase. But the lights and especially the smoke unit never got enough power to work as well as my newer mogul unless I really gave it some power, but then it was moving too fast.
Ok, a toroidal filter is one type of an inductor. It uses the principle of bandpass via self-inductance that causes interference with (or, more correctly, attenuation of) frequencies above and/or below its design points.