I've seen several pics of helpers shoving on the rear of cabooses on the old 3rd division (Salida -Montrose. It was done fairly often on Cerro Summit. I've also got some old Jackson 8mm film of a 3 engine westbound meeting the Shavano with the helper shoving on the caboose. That train was also doubleheaded, which was also not supposed to be done.
Don't really know why they did that. Perhaps there was something in the track layout/grades at the top of the hill which made it tough to get the helper cut out and the caboose coupled back on, so they simply shoved on it.
I've also heard that they had some cabooses set up to be shoved on, with extra sills or something like that. If you've ever looked under a caboose, you'd say, "OK just WHERE do you put extra sills under here and still let the trucks turn..?" Hmmm...
Perhaps it was one of those "the Big Brass hangs out in Alamosa, nobody bothers us up here, let's just do it the easy way...." deals.