I read where he was quoted as saying "Unlike the other two I didn't know how this was going to end but I knew that I was going to need another season to do it". He went on to say that he had to someone at the network and request more time.

You have to keep in mind the reason the train can't make it is because of the snow drifts which means the challenge is not over. Keep in mind Spencer is coming out of Texas so he could face the same problem. Maybe they will both end up stuck in the same snow drift and reunite. How awesome would that be?