Since I was at school I have found travelling on the underground train sexually exciting. The best ever 'underground' event happened a few years ago. I found a seat in a crowded car, and sitting diagonally opposite me is a girl in her twenties with long auburn hair and wearing black slacks.
Before the train sets off she is smiling and waving through the window to a man on the platform behind me. As soon as the train departs, she crosses her legs and starts to move them rhythmically in a way that tells me instantly (as a connoisseur of such things) that she is not just keeping the beat of music in her head. She can't seem to get it right, however, for she crosses her legs first one way, then the other. Eventually she gets a new idea, she uncrosses her legs, picks up from the floor her small backpack, and wedges it firmly into her lap.
Then, with me hardly able to breathe with excitement, she holds the backpack firmly with both hands in front and starts a rhythmic thrusting of her pelvis into it, back and forth, back and forth. This went for a minute or two, with me sneaking as many glances as I dared and wondering how I could relieve my desperate, throbbing member. All the while, the girl was staring straight in front of her, as the rhythmic movement continued without a break.
After a minute or two I noticed that her eyes were starting to become glazed, her face was becoming flushed and, best of all, her lips parted slightly. Eventually she let out a short, audible sigh, and the movement stopped. Next stop she got out, and it was my stop too. I went up the escalator directly behind her, and looking at her behind, and imagining what it was like inside her knickers now, and with my hand inside my pocket quickly came to a massive orgasm before reaching the top of the escalator. The best train ride ever.