My thoughts. Engines 456-463 had maximum tonnage Cimarron to Cerro Summit with the 17 loads. In order to get the 21 loads to Montrose they would have been required to double Cimarron to Cerro Summit. To avoid the double, engine 361 was used to move the additional 4 loads. Of these 21 loads 20 were stock. The non use of 361 as a third engine may have been to save additional switching in helpers at Cimarron and switching out again at Cerro Summit. Also engine 361 would have been required to take full crew and caboose, so might as well run it as a "second section". Thoughts?
I think you are right. Maybe switching but probably a labor-based decision.