John:
I can't say if your scenario for swithching Cumbres is accurate, although it likely varied somewhat. To this point on page 96 of Dorman's Chama/Cumbres book we see a reefer (LCL freight?) and a couple of cars being switched circa 1950's. So maybe the turns were not completely blocked at Chama. On page 92 of Krause's American Narrow Gauge there is a shot of a EB train being made up with the caboose fouling the main. I think there may also be some video I may seen of switching Cumbres maybe by Emery Gulash from Green Frog? Jim Vail has been converting 8mm films to DVD and offering them for $20 and has a couple of sequences of switching at Cumbres on his D&RGW freight DVD. In one scene they are setting out the turn and are plowing the snow on the siding at Cumbres with a MOW RPO that was in the consist before the helper (473) heads back down to Chama. Jim's films are 1960's but they don't capture a complete sequence of switching at Cumbres.
Rod