In the past I said to a girl on the train:
"Hey, er, you'll probably laugh at me or something, but I thought it's worth a shot, do you fancy going out for a drink or something?"
She looked a bit shy at first and said "er, yeah, ok"
I gave her my number and just said if you do, gimme a ring. 3 days later she did!
It turned out to be a quick one night thing but hey can't win 'em all...