Class 56 vs Class 60 on D&RG

Posting on the RGS engines thread has brought an old question back to my mind, specifically, how did the Class 56 and 60 engines compare?
As in the post I was responding to on that thread, I've seen many people over the years put Class 56 in the "early, light" catatgory with the 2-6-0's and 4-4-0's but the great majority of the 56's were around until the "Rich Mans Panic" of 1907, most finally scrapped in '08 but some lasting until 1917. After the last of the original Class 40 2-6-0's left (1895?) the Class 56 were the lightest freight engines on the narrow gauge but D&RG seemed in no hurry to scrap 'em.
I've always wondered if the comparison would be more like that between the K-36 & K-37's where a 37 is said to be good for one more load up a 4%.
Hard data is a bit thin on the ground but here's some to start off with:
Original specs:
Class 56 Cyl 15x18", TE 12,450#
Class 60 Cyl 15X20", TE 13,800#
Not greatly different. original spec Class 70 engines a TE of 17,055#
From D&RG 4th Div. Emp. TT #102 dated 7/20/1919:
Class 56 Nos. 30 to 84 inclusive[1] had the following tonnage ratings(A ratings):
Chama-Cumbres: 85 tons (Class 60 rated 95 tons, Class 70 at 105, class 125 at 200)
Antonito-Cumbres: 200 tons (Class 60 @ 220, Class 70 @ 245)
Hermosa-Silverton: 130 tons (Class 60 @ 145, Class 70 @ 165)
Note that by this date the Class 60 & 70 engines would have had their new boilers for about 6 years and thus are more like C-16/17/19's while the Class 56 were never so rebuilt, I'd assume the tonnage ratings were closer in ye olden days. [Also, for those who have only seen later tonnage ratings, D&RG(W) lowered the ratings on all the 3' gauge engines at some point in the 1920's)
[1] I assume this is corporate inertia in action as all of these had been scrapped by 1917!
So.
Anybody have any have more data or some notion of what the operational difference between the 56 and 60 class engines really was?
Inquiring minds and all that.

hank