"Moonlighting" is forbidden in most full time contracts of employment.
It's not in mine, but I do need written permission for it. Only reason I ever even noticed was that it was quite widely worded, and because volunteering is a fairly big part of my life I wanted it in writing that that was OK before I agreed to take the job.
In minimum wage jobs, though, whether formally allowed or not, it's common practice.
My gut instinct is it's an enthusiast on the spectrum. You see this sort of roleplay fairly often, especially on social media.
It's not hard to end up in that position inadvertently though not checking tickets, obviously*. I've been mistaken for staff on a number of occasions, though I usually do answer queries prefixed with "I'm not staff but..." because most of the time I can answer them and don't want to send people on a wild goose chase.
Most recent time was on a late evening Chiltern, when I was wearing a blue shirt, formal trousers/shoes and a blue soft shell without any branding on it, I could understand that confusion, as it's pretty much Chiltern's colours even though nothing on me mentioned them. I recall having a similar thing ages ago when I was stood next to my sister by a Virgin Mk3 set at Preston chatting, me in a similar outfit to the Chiltern occasion, her in a red puffa jacket, we must have looked for all the world like Virgin and First North Western staff, and got (and answered) a number of queries.
* Other than if someone asks me if this is the train to X and there could be an operator specific ticket issue I normally ask if they'll show me their ticket so I can give accurate advice.
