Taking another look at the slides from 1966, I see that the mid-train helper was 488 rather than 492. Sorry for the mix up. That day 492 brought the eastbound train from Durango to Chama, and the next day 492 ran from Chama to Gato and helped 498 from Gato to Falfa before running light to Durango.
Here is the REAL mid train helper on the middle level of the Whiplash
488 on the upper level of the Whiplash
492 leading 498 across the Piedra River bridge between Gato and Arboles
492 and 498 between La Boca and Ignacio
492 and 498 climbing out of the Florida River bottoms toward Falfa