In Portland, a far too frequent cause of MAX (the light rail) service interruptions is due to car/train collisions. See the following links for a small taste:

http://koin.com/2014/04/01/1-hurt-car-max-crash-portland/ http://www.oregonlive.com/portland/index.ssf/2013/06/max_col... http://www.oregonlive.com/portland/index.ssf/2015/05/max_blu... https://www.reddit.com/r/Portland/comments/3d5i9g/saw_an_ora...

There may be more to it in Portland than "at grade", but that certainly seems to be a large component of the problem.