"Excuse me if this has been answered before, how far south could a 480 or a 490 class engine go on the Santa Fe Branch?"
As the information I have been posting pertains to several different years, and I have filed my copies in unknown locations, I can not say for certain what year this occured: A 480 class engine, I believe it was 483, went to Barranca to assist No. 426 that was having engine problems and took the train to Antonito.
The resctrictions on operating the 480s or 490s on the west end of the line would not necessarily be a result of the rail size. There were sections on the east end that had lighter rail than the west end. Some sections between Antonito and Tres Piedras had 45# and 52# rail. From Barranca to Santa fe the lightest rail was 57#.
Jimmy