I've done that exactly twice. The first time was just a hand. I put my hand between her thighs and got under her skirt, managed to get her panties aside and played with her labia a bit. She froze at first but I knew she felt it. When she started looking around I slipped my hand back into my pocket, she didn't catch me because she wasn't right next to me, I was basically reaching between two people who were wearing heavy coats.
The second time I was pinned up against the woman and I went further, she was an older woman, well, older than me at least (I was 23 and I'd guess she was like late 30s early 40s.) wearing a tight pair of yoga pants.
The lights in the car weren't working too well so I figured I'd be able to get my cock out without anyone seeing. I got myself hard and pressed it between her ass cheeks. She didn't seem to notice that, but I got cocky (ha) and put it between her thighs.
Now this is why I don't do this anymore. Once I got fully between her thighs I felt her thighs tense up. She looked down, though I don't imagine she saw anything, I mean I'm not small per se but I'm definitely not big enough to be visible between that ass and those thighs. She did however start holding her coat in front of her waist.
I didn't move A because I thought foolishly that maybe she hadn't actually noticed yet and B I still wanted to salvage this somehow and get off.
While I was frozen like that I felt something touch the tip of my cock, I saw her arm moving and 2+2== she was trying to feel what was between her legs. I was scared shitless at this point, I was imagining her turning around and decking me, or digging her nails into my cock, or reaching between her legs and tearing my nuts off, or just fucking screaming and everyone seeing me doing this to her.
Instead I feel her fingertips rubbing the head of my cock, kind of teasing me I guess? then she squeezed her legs together and started kinda slowly moving her hips. Her hand disappeared for a second and then I felt what I assume was her palm, all wet, rubbing the head of my cock. The whole thing felt amazing.
I came into her palm and she really quickly turned around to face me. She grabbed my dick with the hand holding her jacket and licked my cum off her hand. She whispered to me "I thought about ripping this thing off, you're lucky you caught me on a good day. I let you have your fun, now you can return the favor at my stop or I can yell right now and let everyone see what I'm holding."
Her stop was one after mine so I skipped mine, she dragged me into the restroom and gave me a crash course in eating pussy I'm actually pretty grateful for, then she let me go and I walked the rest of the way home. Never did anything like that again, I got supremely lucky and whoever's feeling you up is getting even luckier. If I had to guess it's probably the same person or people every time because they recognize you and realize you won't do anything.
I feel ladies up on the subway all the time. They never suspect another woman.
Are you in NYC?
One of my earliest shockers when I was around 10 riding the subway from Manhattan to Brooklyn, a long ride, I saw a woman clasping a straight, shiny razor, opened. Her frowned eyes were darting around. All I can say is that, suddenly, I was glad to be only ten years old.