manufacture some kind of situation where you'll have to talk to her, i suppose. anyone just coming out with "hi there!" to a stranger on public transport will generally seem weird. although i think it varies between countries how much you can actually get away with in that respect.

often if you can make a passing comment complaining about the state of the bus or how it's late or whatever, that can be a good opener; the worst response you'd generally get to something like that is a non-commital "yeah". and if she's interested back (hey you never know) she might take the opportunity and strike up an actual conversation.