A colleague of mine had his own girl on the train.

He took the same train into work in Central London each day.
He would often comment on the attractive girl he saw, and how he never spoke to her.
Part of him didn't want to spoil the mystique.
He was quite happy to travel on the same train as her, and walk behind her from the carriage, along the platform and up the escalator, never exchanging words.

This was because the most erotic thing about her was the way the way she walked.
With each step he could just catch a glimpse of her stocking tops revealing themselves from beneath her skirt.
This peek-a-boo journey from train to street level was what the girl gave him, and he found this transfixing enough.