I love chatting up girls on the bus sometimes.
Other times it's like you say, gut wrenching. I've been sat right beside chicks & imagining the perfect conversation playing out in my head, yet I am sat right beside her, imagining it
I usually ask them if they know how long it takes to get to our destination (They know you already know) & then go from there "so have you any plans tonight/weekend/christmas/easter?"
Once you get started there may as well be nobody else on the bus even if its jam packed.
I actually love packed buses because then you get to hand pick the hottest girl there & ask if she minds do you sit down, which gives you brownie points in itself for being polite because just now that i mention it i don't recall ever seeing anybody ask someone if they can sit down, usually people just plant their ass on the seat because well, it's a bus they're "allowed" to. But if you ask you not only show you're a gentleman or whatever blah blah, but you have effectively broken the ice too.
Try it out.