Some of my engines like the TCC GP15 aren't specific to one particular operating scheme, but are used in several under the guise of "leased power".  Other engines are specific prototypes that I wish to model, regardless of whether they fit in any of the operating schemes or not.

 

I will run each of these engines on the layout from time to time, whether as part of "visiting power" during an operating session, or simply just to run them around the layout.