I used to have it very often when I was regularly volunteering on the Mid Hants, but also more generally. Although my heritage uniforms are circa-1960, my 'regular' clothing all loosely 1920-1960 in style, and as such my attire never bears any substantial relation to that of current railway staff I have often been approached, sometimes even with Railway Staff very close at hand. I once even ended up being issued with a priv rate ticket!!
A memorable instance was at Surbiton on a Saturday evening. I was still in my heritage uniform, with my Guard's bag, BARDIC, cap under my arm. For the whole time that I was there I was being asked the questions instead of the SWR staff member about ten feet away who was in the standard Hi-Vis based uniform. This, and other instances, has often made me wonder whether there's something to be said for railways returning to a more traditionally-styled uniform, albeit with concessions to modern requirements.
Had a few instances at Clapham Junction, both in and out of uniform. One of the ones in uniform a Lady saw me coming out of the subway and stared at me as it she'd seen a ghost (The oil lamp probably didn't help that impression!). Another time, when not in uniform, I saw a member of GTR staff struggling to get a wheelchair ramp out because he didn't have a carriage key; He was somewhat surprised when I wandered over and handed him mine!
Yet another one was at Alton, where SWR connects to the MHR, where an SWR-contracted cleaner asked me if I was an inspector! I've also given directions to replacement bus drivers several times, and at least one of those times I was taken to be staff.
I think one reason why I often get asked questions is the unfortunate way that I conduct myself when on the network, and it's only since being in a relationship that I've noticed this (!!!). When coming into my destination station I will often stand by the door in a not-dissimilar manner to a member of train crew, watching through the window of the door. Then, upon alighting, I'll often stand back and look around the platform as a guard might, then watching the departure before walking off. I hadn't noticed this much myself, but now I realise it can make me look more like staff.
One time, when in uniform, I felt especially guilty for not retaining the mindset of the average passenger. It was a busy Friday night at Clapham Junction, with the platform I was on close to overcrowded and several passengers getting far too close to moving trains, with a curved platform meaning that what staff were there couldn't see the length down to where I was. In my mind I was worried about a fatality occurring so set my BARDIC's filter to red and kept my whistle close at hand, essentially preparing myself to attempt to stop traffic. In the end I realised this was a silly and potentially dangerous mindset, so I removed myself from that situation.