I don't know the date that the tank "poped" off, but I notice in my pix of westbounds between Alamosa and Chama in the 60's the empty tanks are at the headend behind the engine. I always assumed that was to simplify switching at Chama. But now I'm wondering it this was a change in train consists after the problem at Lobato. The eastbound loaded tanks seemed to be about anywhere in the train going down the hill from Cumbres to Antonito.
JBWX