How can I strike up a conversation with a woman on the subway who is wearing earphones and listening to music?

It seems like a lot to ask of a woman that she takes off her earphones for me to find small talk or an inroad to talking with her. I can start up a friendly flirtation on the subway in some occasions, but the earphones are an obstacle. Yes, I realize some women use earphones as an excuse to ignore guys and keep them away. I'm not trying to bother women who clearly want to be alone... but there are certainly some who don't mind being chatted up on the subway if it's done respectfully.