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There are three capacitors. There's a through-hole ceramic across the frame, the electrolytic I mentioned (SMD can-type), and a small chip cap across the motor. You can see them on Mike's video from 2:11 to 2:22. That last one is likely most of your EMI suppression. You've got me on purpose of the electrolytic; given its small size I would not be surprised at all if it has a 15V rating (maybe 25V?) and is why they say "don't do that!". I'll check tomorrow.
Good question. I don't know the MOV's rating since it is house-marked; sadly there is no standard for marking these things. What's clear at this point is this is the reason they say don't use on DCC, they know the specs and that the MOV happens to be under-rated for DCC voltages. But you're right - overkill. Not quite sure what the objective is, maybe they're protecting themselves against layout operators doing something stupid like setting the DCC voltage high (O scale = 20V). In that case the track cleaner car is going to be the least of their worries. :shoulder-shrug:Anyway, not a full short, at 2Ω it would have been just barely past the 5 amps of the booster, so it didn't trip. Interesting.
The MOV is on the AC side of the bridge.
So what's the bottom line now? I have been running my units on DCC w/o a decoder for nearly 2 years with no issues. Have I just been lucky?