Yes. SBB train conductors (as I am) can look-up realtime train schedules from all railways and verify if your train was indeed running late. Simply indicate the original itinerary.
@StéphanieB1 ´s answer is absolutely correct, a confirmation is required according to the conditions; however, from pratice, I can tell you that for the Zermatt to Visp route it is unrealistic to get a confirmation as, in case of a delay, there are hundreds of passengers wanting a confirnation and the conductor would simply run out of paper and time 🙂
On the other hand, trains from Zermatt are rarely running late - be optimistic.