I have seen them substitute a K-28 for a K-36 in seasons where they ran 4 trains per day. The train for the K-28 is a bit shorter.
Sometimes they will even run 2 sections of a "single train" each pulled by a different locomotive. If they have 2 operable K-28's they can divide the full train into 2 sections.
As previously stated I do not believe they have any plans to restore a k-37.
They would love to have another K-36, as noted by the attempt to trade 478 for 483 with the C&TS.
Presently the 483 is the only K-36 which is not on an operating/maintenance schedule.