In pursuing my D&RGW Employee TT collection that starting in 1937 and continues through 1968 I have the following information:
From 1937 to 1953, the rule was as follows: "9-G. K-36 and K-37 class locomotives must not be double-headed across bridges 319.95 (Cascade Creek) and 339.78 (Wolf Creek)." There is no restriction on double-heading with anything smaller. The heaviest load permissible would have been a K-28/K-37 combination. Jerry's pic shows a K-28 helping a K-36, so no foul there.
By 1959, the rule was modified to: '19-C. K-27, K-28, K-36 and K-37 class locomotives must not be double-headed over bridge 319.95, and K-36 and K-37 class locomotives over bridge 339,78, Sub-division 11." So, no more double-heading at all over Cascade Creek. The restriction on Lobato remains the same.
By 1963, the rule was modified again to: "19-H. Locomotives must no be doubleheaded over Bridges 319.95 and 339.78 Sub-division 11 or Bridges 452.42, 471.23, 477.81,and 489.88, Sub-division 12-B and must not be operated over these bridges unless separated by at least 100 feet. This separation must consist of lightly loaded equipment. It is not permissible to operate two locomotives over these bridges with only a flanger between them." So, no double-heading across any bridges. An interesting point about double-heading restrictions on the Silverton Branch, as that the bridges named are virtually every bridge across the Animas River.