speaking of derailed...
I think if you want to initiate contact regardless of whether it was accidental or on purpose is say 'no problem. Can I help you at all?'
If a girl is interested she may respond...
'yes, this trolley is all wobbly, I can't seem to get it to go in a straight line!'
or
'well...actually, I was looking for x, do you know where that is?'
In either case, a conversation can follow and she thinks you're a gentleman.
To be honest I have never done this kind of thing on purpose. But maybe some girls do, who knows.