it's really easy to sue property owners and get away with it, people do it all the time. 9 times out of 10 they have insurance for that type of thing. the person suing asks for some ridiculous amount (like hundreds of thousands) and the insurance company gives them a nice settlement, usually 20 or so grand. sort of like getting in a bullshit accident or getting hit from behind in a car. if they even do sue, the property owner get's higher insurance rates but that's pretty much it...
I honestly never heard of anyone suing for skiing, skateboarding, etc. Usually there is someone filming and if that is brought to a judge, they could issue a warrant for the footage, in which case I could see you being fucked and not getting anything. I could imagine it being pretty hard to prove that a someone hitting an urban rail was doing it without intent and by not knowing the risks involved.