Was that sarcasm? A person without schizophrenia usually does not walk on train tracks and in case they do, they usually are aware that trains can pass by. So now while also other people sometimes get hit by trains, in his case his mental condition might be related.

So why was he walking on a train track? Is this uncorrelated to his diagnosis?

It was indirect cause though, if it wasn't for his schizophrenia, he probably wouldn't have become homeless and aimlessly wandering through the states which is when that happened.