OK. True story, but from a long time ago - and I'm talking almost a quarter of a century (!) but, reading some of these, I just had a vivid flashback to it.
I was working in North London and had to commute up the Northern Line.
In the rush hours, heading North, this is horrendously crowded until you reach probably Euston.
One morning, I was lucky enough to grab a seat at Waterloo and went into the commuter trance, as you do, whilst the carriage fills up and the atmosphere gets increasingly hot and stuffy.
After a few stops - around Tottenham Court Road I reckon, I felt some contact. Or, to be more precise, I felt a youngish woman standing and holding the bar adjacent to me, pressing her, shall we say gusset, against my knee.
I retreated slightly, assuming this was accidental and slightly awkward involuntary contact. Only for her to press harder and rub herself slightly but noticeably against me. I glanced up and she made eye-contact and grinned at me.
Maybe I should have felt flattered, but it felt decidedly weird at the time and I was quite relieved when she got off 1 or 2 stops later, with just a quick backward glance at me.
If the sexes had been reversed, I suppose it would have been regarded as far more inappropriate.