This is actually a tough one since she's fully engaged in work when you see her.
She's probably not going to be open at all to being hit on or even chatted up while she's working.
But... I like wilked's plan a lot.
The only thing I'd tweak is don't hit her with the note too fast - build up at least a week or two of daily smiles and getting her to recognize and remember you first so asking her out seems a bit more natural.
One other suggestion to help her remember you, and it's a bit cheesy and romantic I admit, but just before boarding the bus each time, stuff a sock in your crotch.