Even if grinding does occur on the rails of the bridge, as long as it doesn't cause real damage, and if it's not done while pedestrians are walking across the bridge...who gives a shit. Likewise for skaters skating across.
Believe it or not, I think most of these kids know better.
Briv -- All I know is that if I was 9, 10, 11, 12, 13,.. -- that bridge would be an undeniable attractant for at least mildly crazy behavior. I would be there with some of my competitive friends trying stuff which in the absence of others might only be mildly damaging to us. However, mix-in someone who is unprepared to meet a speeding kid and unable to take effective protective measures and you could have a tragic accident.
I don't think that I was that wild and crazy compared to my compatriots -- but between 8 and about 14 I:
1) drafted behind vehicles while riding a bike down long hills to see just how fast you might be able to go with less air resistance
2) sprayed water on snow to make an icy launch ramp for sledding down hills
3) raced against my peers down paths in a park with primitive skateboards which we built
kids and the lure of speed are inseparably linked
So I would challenge your statement about kids knowing better -- they don't
Not that they mean to be dangerous -- they are just kids
And some of us are still kids at heart -- I for one still enjoy running along with my dog down a local hill in our park after / during the first snow of the season and often ending skidding and sliding on the snow together down the last few feet -- with a lot of barking and laughing