In the 20s and 30s until the end of WWII, there was more traffic on the old Third Division out of Salida to Gunnison due to the heavy coal traffic out of Crested Butte. In 1930 the CF&I enlarged and electrified the Big Mine at Crested Butte and built a new screening plant at Salida that was connected to the barrel car dumper. 80 and 90 car coal trains on the Crested Butte branch were common in the WWII era. You can see this in Jimmy's dispatcher postings. In 1940, the D&RGW rebuilt the bridge at East River on the Crested Butte branch so they could use the K-37s on the branch.
The first use of a K-37 on the Crested Butte branch was October 28, 1940. With all that coal moving over Marshall Pass and the increased output from the Monarch limestone quarry, the D&RGW needed more motive power out of Salida, that is why you will see K-36s up there in the late 30s and 40s.
Denver and Rio Grande Western Dispatcher Book
October 28, 1940
Crested Butte branch extra 474 with helper engine 498
Conductor C M Purceil
Engine 474 Osgerby
Engine 498 Rusk
Depart Gunnison 11:15 Am with 3 loads 85 empties
Mile Post 292 25 minutes unload cinders
Arrive Crested Butte 1:50 PM with 2 loads 86 empties, 4 hours 40 minutes mine work, 30 minutes inspection
Engine 498 depart Crested Butte 2:15 PM light for Gunnison
Extra 474 depart Crested Butte 6:55 PM with 70 loads