Yes, that was the planning mill. Up in the rafters were big boxes full of car department records. Occasionally, one would fall and scatter itself around the inside. One day Dave Peterson and and I scavenged up a half dozen big green trashbags full of car dept stuff. By and large is was standard gauge stuff of little historical value, but occasionaly you find a car dept inspection of an outbound NG train.
The planning mill and roundhouse were the last buildings to go. Uncle Pete torn them down just before Christmas in 1997. I happened to be in ALA picking up some supplies for the Christmas Train we were running in Antonito the next day and went by the yards. The planning mill was already flat and a dozen locals were picking through rubble picking out used bricks and loading them into pickup trucks. Across the way, the windows and doors were knocked out of the roundhouse, and big steel cables were looped through the window openings to be pulled by bulldozers to bring the building down. It was gone the next day. Truly sad. The only remaining D&RGW structure from the Alamosa shop/roundhouse complex is the sand tower, made out of a tank car.