Found this sub recently and it's actually the perfect place for this story.

A few years back I was at a festival, one of those events big enough they have to put extra trains on. It was on the way back and we were packed in, basically as busy as you'll ever see a train in the UK. I had my back to one of the upright poles and my rucksack was being held against my chest. I literally couldn't see anything down there. I became aware of something pressing into my crotch, I'm pretty certain it was a hand that was gripping the back of the chair in front of me, I tried to move back but the pole I was against wouldn't let me but then the hand slid up, problem solved.... then it slid back down again for a bit, then up again. I assume they were testing the waters as it slid back down again and stayed there. I very likely could have twisted one way or the other just enough to get out of reach but at this point I was curious.

It stayed that way for a while, they seemed to take me not moving as silent approval and I eventually felt movement. I think they were flexing their fingers so the knuckles pressed into my cock a bit more and I eventually pressed forward a bit sandwiching their hand between my crotch and the back of the chair.

The hand slid back up again suddenly and I assumed the game was over but then it slid back down again only they'd turned their had round. I felt soft gripping motion as they loosely grabbed me, we stayed that way for a while and I gradually got stiffer and stiffer. At one point they slowly helped guide my shaft till it was upright inside my pants. The rest of the adventure was them subtly gripping and ungripping my cock while I pressed back and forth into them.

That was it basically, it got to my stop and when it became obvious I was moving off the hand snatched back. I never saw who it belonged to, don't care either. When I got in I wanked myself silly and still think about it regularly.