A lot of the modern fairground rides appear to run off AC from standard industrial packaged gensets, the days of Gardner powered DC sets in the backs of the lorries are pretty much gone. I don't know if they use transformer / rectifier packs for the older rides (but I know someone who might know, I'll ask).
At the Old Warden rally there is a big line of Showman's engines in front of the fair, some of which generate for the traditional rides. In the case of the Steam Yachts, the engine is only providing the lighting as the ride is powered directly by its own directly acting steam "engine".
It's taken a while to get to this point because in the early days of steam preservation some of the Showmens engine owners were keen to get them generating but "on the night" it turned out in some cases that while the engine may have been able to trundle around the rally field it wasn't up to running at full load for sustained periods, leading to breakdowns which hit the ride owners in the pocket and meant they were subsequently very wary about taking power from preserved engines. Even now there is usually a diesel set round the back just in case.
was at Old Warden again this year.