Generally it is weather related problems though. The tracks are a fixed width and heat makes them expand so the train wheels could technically fall off and cold makes them contract so the axles on the train are too big for the tracks. Also this leaves on the track stuff, as you probably know there's little traction between rail and wheel as both are smooth, when this is coupled with leaves inbetween them braking distances are increased therefore speeds are decreased leading to delays.

I'm not their sole employee as I have no soul! ;p