Just to follow-up on this from what the others said-I believe the idea is to get 463 to Antonito as much as possible since it is best suited for that run. The lighter grade means no train is too much for it between Antonito and Cumbres. The downside is that it needs help on the Chama-Cumbres portion eastbound when it returns to Antonito. In essence it saves wear and tear on a K-36 from Cumbres to Antonito and the next day back to Chama. If a helper is needed the K-36 from Chama is only worked hard for about 60-90 minutes and it returns to Chama.
Greg