I'd guess less than by not having any flowers at all. Where I live, most (non metro) trams drive pretty slow, like 30km/h in long straight parts, so unless they get between the wheels and the rails I doubt anything would really hurt the bees.

Trams on average do not drive too fast. I would imagine that bees would easily be able to evade them or even be bumped out of the way.

I didn't think of that. That would require scientific testing. Good point!