This happened about 20 years ago. I was working in London and had been home to Scotland for a long weekend. I had taken the overnight train from Edinburgh back to London, and settled down in my seat to try to get as much rest as possible. The train wasn't packed but there were a number of people scattered around the carriage, and moving up and down the train to the buffet car, etc.
I was already horny and the movement of the train was making matters worse. I shifted around in my seat trying to put it out of my mind and get some sleep. I was used to having a cum at least twice a day and had not had a chance to do so that night before getting on the train. The rhythmic motion of the train seemed to be sending vibrations up through the seat and into my pussy, which I could feel getting wetter by the minute.
At last I could stand it no longer, and having draped my coat over me like a blanket, I rested my feet on the seat opposite and slipped my jeans and knickers down to my thighs. My pussy was already soaked with juice. In those days, I had a thick bush and the hairs on my labia were already wet. My cunt lips were swollen and sensitive, and my clit was hard and throbbing.
I was terrified that someone would guess what I was doing, but I couldn't help myself. I had to cum. I rested my fingertip on my clit and circled it gently, as anything more vigorous would be too obvious, stopping whenever someone passed by.
I could feel my juices running from my cunt down my crack and knew I was only moments from orgasm. I buried my face under my coat as if I was asleep, so that the inevitable moan would be muffled. I'm always very noisy during orgasm and although I can keep quiet if I really have to, it was going to be difficult.
I gradually increased the pressure on my clit and minutes later one of the most powerful orgasms I had ever had washed over me like a tidal wave, and I squirted all over the upholstered seat beneath me, a common occurrence in those days, sadly much rarer now.
I sat there for a few moments getting my breath back, before slipping my jeans back up and moving to another seat, hoping no-one would notice the wet patch on the one I had just vacated.
Since then I have brought myself off on trains, buses, boats and planes. There is something about surreptitiously wanking in public that gets me going every single time.