I have an old inverter for the car (plug into cigarette lighter to get AC output). When I use a tape player on it to record, I get a buzz when I play back the tape, which suggests to me the power source is 'dirty'. Since the device is old (bought second hand), I suspect it'd be "square wave".
It's on an electric train, either locomotive or self propelled. I'm more concerned that the power couplings between cars aren't loose. Also, I believe the car power is supplied at 440V and I don't know how good they are at knocking it down to 120. I think the outlets were intended for electric shavers or vacuum cleaners when the train is being serviced, neither of which is fussy about smooth current.
There was just an article in the NYT about commuters finding outlets in terminal builidings for their toys. Apparently the Port Authority is ok with that. I had to do that once and left my device for a minute, and came back to find the doors locked to the waiting room. Lucky I found someone to unlock them to get the device (a camcorder).