Topics that are acceptable:
1. "What do you do for a living?"
2. "What's your reason for being here?"
3. "What's your name?"
4. Anything immediately common to your circumstance.
5. Responses to any of the topics above.
When you're on a train with someone, it's likely you'll run through all acceptable topics very quickly. The woman on the train said that she was a pi farmer. She spent her time mining pi for interesting sequences. Long strings of seven and so forth are nice. She's also found strings of numbers that make little pictures when encoded into standard graphics formats. What she's really hoping to find is either a copy of the entire text of Hamlet, or the DeCSS script.
Then she told me a story about two friends of hers, a couple. They were on a road trip, and they were very, very tired, but couldn't afford to stop the car. So they took off all their clothes and kept driving. The theory was that the fear of embarassment from getting pulled over or having an accident would be enough to keep them awake. In response, I told her that the Helvetians, before leaving their mountain homes to go to war, burned all their farmlands, so that they couldn't come back. She mentioned that Mutually Assured Destruction is probably what got the world through the Cuban Missile Crisis alive.
Upping the stakes so that you wouldn't accept failure.
What's that got to do with pi farming?