Following is from the D&RGW:
D&RGW Alamosa Division Time-Table No. 120, effective June 6, 1937
Special Time-Table Rules
31-B
In operating three engine trains out of Cimarron westbound, and out of Montrose, Sargent, and Chama eastbound, use two engines on head end of train and one engine on rear end, the rear egnine either just ahead of caboose, or drover's car when latter is ued, except, that engines will not be double headed over bridges between Gunnison and Cimmaron--must be at least five cars between engines in this territory. On two engine trains out of Gunnison and Villa Grove, eastbound, place them on head end of train. double heading between Salida and Marshall Pass is prohibited. Place one engine on head end, cut one engine into train about twenty-five cars from head engine and place one engine on rear end, ahead of caboose.
Note: D&RGW rules prohibited a helper pushing on the rear of a caboose, but there are movies and photos of it being done.