I've noticed all the train stories from UK posters so I thought I would share.
This happened way back in the early 1990's. I was traveling on a very overloaded London Piccadilly line tube train. It really was rammed to the brim. Back then the trains were a bit smaller than they are now and I was standing just in the doorway. In front of me was a young woman in a very tight skirt. Anyway I was a very horny 16 year old at the time so being pushed up against an attractive woman I of course got a raging erection.
Now the thing is she must have been able to feel that I had got hard we were pushed together so tightly. I actually tried to move back a bit but she pushed her rear into my erection. As the train moved she was grinding against me more and more and it did feel really good. I think if it had gone on I would have cum. But after about three stops we came to a main station and quite a lot of people including her got off the train. She gave me a wink as she left.
Nothing like that has ever happened since on a train.