A late train home on a Friday or Saturday night has even more x-rated sights on display. Bolstered by a few cocktails, inhibitions are lowered, people are bolder and less bothered about an audience. You may see a hand disappear under a mac and people begin to believe that lap dancing is acceptable.
But on the other hand. Sometimes I will glance up from my Metro to see a couple sharing a secret smile across the carriage and you can tell that no matter how pressed up they are into somebody else's armpit, they really can't see anybody else. Recently I saw a young tourist couple standing in the crush of commuters and the guy gently brushed his girlfriend's hair from her eyes and kissed her cheek. It was a genuinely tender moment that made me smile instead of feel sick.
The cutest moments though are when I see old couples together on the Tube and the old man will be a gentleman and ensure his wife gets a seat and she will smile at him gratefully. Makes me believe in love. Even on London Underground.