I'll bet the list for accessories that CAN operate on DC is longer than the list that can't use DC. Obviously the vibra-motor mechanisms (cattle platforms, rotating searchlights, wig-wags) need the alternating voltage. I think there's a crane that requires AC voltage though not because the underlying electro-mechanism requires AC but because it uses one polarity to make the motor go one way, the other polarity to make it go the other way.
I think of all the 12V DC mechanisms in the automotive world - power locks, windows, seats, mirrors, you name it. They go up, down, rotate CW, CCW, fast, slow, momentary operation, continuous operation, etc. All the types of motions that train accessories use.
If done today, I'd think the vibra-motor type accessories would use a DC motor with an eccentric/unbalanced weight on the shaft like vibrators first felt in pagers, then cellphones and now smartphones.
Footnotes
(1) there was a recent thread about DC vs. AC where someone's grandkid said he'd rather have the gateman solenoid operate on AC than DC. Why? Because the AC buzzing made it sound like the gateman was talking! Ahh, to have a child's imagination!
(2) there have been occasional threads suggesting some vibra-motor accessories operate better using pulsed DC (using a 10 cent diode) to drive only "positive" polarity power. As to whether pulsed-DC is not "real" DC or is still AC is open to interpretation. It's interesting that recent command-control engines from both of the big O gauge manufacturers can operate on DC. While perhaps a theoretical exercise, it is now practical to have a DC-only O-gauge layout...both trains and accessories. For the vibra-motor accessories, low-cost eBay modules can generate pulsed DC from a 12V DC supply.
