Hate to burst your bubble here, but it took that train over 18 hours to get from Alamosa to Durango. whereas the San Juan did it in 9 hours or so. My friend Eldon Morgan ran 478 from Alamosa to Chama that day with Jim Shawcroft firing (I ran with Jim on the C&TS for a few years). They were called in ALA at 530am and didn't tie up in Chama until 335pm. They then deadheaded back to Ala. The track was getting pretty rough then, and the last thing you want to do is try to run a loaded freight train over rough track too fast.
MY guess as to why they ran the train all in one day is they had no eastbound traffic to run from Durango, and they wanted the engines over in Durango ASAP. The Silverton Train started Memorial Day weekend back then and 478 and 476 were going to be the regular engines that year. 473 missed her date in the Alamosa shop the previous winter when her Dec 1967 trip east with 497 was cancelled because of bad weather. 473 sat out 1968 as the spare engine and went east as light power after the Butch Cassidy flick was done in Oct 68.