Is it well known, or well-speculated? I don't believe it was ever definitive that the train accident caused the onset of his illness, but I might be wrong.
As to the assassination attempt, no, that's not safe to say. That too was speculation, but again not proven, no evidence was produced that firmly linked the accident with an attempt on the Tsar's life. In fact, the accident seemed to be caused by the train travelling too fast for the condition of the rails it was travelling on.