If you pay attention, you'll note that UP hardly ever includes a diesel with it's steam locomotives. On the few occasions when they do, there are usually good reasons for it, such as mountainous terrain, heavy trains, tight schedules, or all of the above. They also occasionally move one of the heritage diesels to and from displays that include the steam engine.

They do not put a diesel on every steam train; in fact, far from it. In this, they are just about the last of the steam operators that don't.