Locomotive values

It seems that in the past most people have indicated that the k-36's were a more reliable and easier to maintain than the K-37's..this raises some questions in my mind. I was looking through some photos and came across a picture of the 497 and 498 on their side near Tangle Foot curve. The photo was from the mid sixties my question is why were these locomotives repaired and put back into service when there were very few trains running at this time? Also if the K-37's were so unreliable why did the Grande operate them until the end, from a lot of the photo's I've seen on the Alamosa to Durango line the road engine was usually a K-37 with a K-36 helper. Was it a practice used by the Grande to have a more powerfull loco as the road engine or did it really matter? And lastly from an engineers standpoint which engine did most crews prefer a K-36 or K-37? Just some questions, hopefully I can get some views---thanks.