Craig,
This is only speculation/conjecture from what I have been told and read here and at other sources over the years.
In 1955 the handwriting for the remaining ng was clearly on the wall. The Valley Line had been gone for 4 years (from Hopper to Mears Jct.); The Marshall Pass line was being scrapped. Plans were in place to convert the Monarch Branch to SG (thus eliminating Salida as a ng terminus); The pipe business was peaking; Plus (and maybe the biggest factor)- Perlman was still with the D&RGW.
In 1955, the Rio Grande still had 26 steam locomotives on the ng. 1 - C-16 (#268), 2 - K-27's (463 & 464), 3 - K-28's (473, 476 & 478), 10 - K-36's (480-489) & 10 - K-37's (490-499). This was plenty of power for the remaining San Juan extension.
I also understand the damage to 485 was pretty substantial. The frame was broken and there was damage to the cylinders as well. I have been told that the engine was pretty much cannibalized before being scrapped. This also happened with 490 about the same timeframe.
If I left out any facts or am off-base, maybe Earl can step in and correct/change.
Thank you,
MAS