I've seen people use them on 377s even though they have big notices on them saying they're not for public use (apparantly they are only earthed when at a depot). If they aren't earthed when the units are running I'd have thought the sockets should be covered up to prevent unauthorised use.
Hmm, not sure what we mean by not earthed? surely the 'hotel power' to AC, lighting etc would have to be either bonded from neutral to the train chassis (as is usual for uk building wiring), or else all breakers would have to be 2-pole ( as used in many mainland european countries). The issue of touch & step potentials would apply to fixed equipment & wiring on the train not just to sockets?
Either way, it's probably a bit superfluous, since the majority of equipment used by passengers (laptop & mobile chargers) are Class II (double / reinforced insulated), in which case even if it has a connection to the earth pin of the 13A plug its only used for the noise filter & doesn't serve any safety function since there no metal case.
From other threads, I think the issue on older trains not originally designed for passenger socket use is that the voltage is unregulated, so might damage some equipment if there are votage spikes. However most laptop/mobile chargers are typically designed with quite a wide voltage/frequency range so it would ahve to a long way out of limits to cause a failure.