This is a real experience has stayed with me (M) for a few years, and I've even talked it over with partners. It's like the opposite version of a train scene from a manga comic, this time with a dominant woman.
First, a bit of preamble:
In my first year of college/university I commuted on the subway. I should say now that I was a late bloomer, kind of skinny and shy, and not used to attention from girls.
I would usually spend the 20-minute ride buried in a book and with noise-canceling headphones, oblivious to everything. I could never get a seat during rush hour, so my favorite place to stand was with my back to the recess of the door linking one car to the next.
The car was always busy, so this nook was a useful place to stand without shifting around for other people. Well, on one morning I'd be trapped in that position while a woman grinded herself against me and almost made me cum.
On that day the train was especially crowded. Now where I'm from, a train being crowded doesn't mean that people will be squeezed in together unable to avoid touching. Even when things are packed you still have a few inches to yourself, and people will naturally give each other room.
At least that's what you would expect.
The first thing that grabbed my attention was the sensation of something firm but supple pushing up against my crotch and then drawing away. Reflexively I assumed that the woman in front had backed into me to make space for another person, and returned to my book.
Then it happened again, a little stronger and for longer, with a slight sideways motion. A wiggle, let's call it. I looked up and took her in for the first time. She had long, cascading black hair, a small waist, broad hips and wore a sophisticated floral scent that was now all over me.
She was about my height, dressed entirely in black, with a shiny leather pencil skirt in constant contact with my jeans.
She pushed again, and I could sense the gentle friction of the material and feel her ass pressing into me and then sliding up and down just a little.
Now my mind was racing. Most of all I was trying to work out if she intended what was happening. I glanced around to see if any was watching, but nobody had noticed.
Should I have tapped her on the shoulder? I didn't want to make a scene, definitely not if this was all unintented. In the absence of a decision I just froze, clutching my book to my chest.