Agreed, sometimes one gets that irritating doubt pop up. The sort where you know it's the right train, but sometimes the brain will go 'hold on, is it?' and checking with someone else already on the train can be reassuring.
Bit late if the train has already left, if it is the wrong one, granted but it happens. It's just a quirk of the brain sometimes! I got on the wrong train in East Yorkshire once, thankfully it wasn't an issue as I was on a rover and the train was going to a place I was valid to. It did rather ruin my plans though, and to this day I haven't done that chord off the ECML towards Leeds. I'm having a dumb moment and I forget the name of the junction. Wait, that's it, Hambleton.
I made that mistake over 15 years ago, and it will forever annoy me. All because I joined a 185 at Brough instead of waiting the extra few minutes for the Northern service I was meant to be joining to Doncaster!