I believe that Randy Babcock (D&S CMO) detailed the railroad's plans pretty well in his post from June of 2016. The plan is to operate 4 K-36s and eventually, 3 K-28s, for a total of 7 locomotives. With that number, they
could potentially have one of the 7 engines undergoing major work for up to 18 months at a stretch without any effect on service. Remember however, that a 1,472 SDI is not a major re-build, it's a boiler inspection, and if that's all a locomotive needs, the D&S could get those done in a lot less than 18 months.
The plan did not mention any use of K-37s at all.
Remember, even during the height of the summer season (June to mid-August), the railroad is only running 3 trains a day. They can get away with 3 locomotives, and if they can keep 5 operable, they'd always have at least 2 spares.
Click here to read Randy's post
/Kevin