I found this definition of operations today on Ron McF's Gulf, Colorado & Santa Fe blog. Ron attributes this statement to model railroader Bob Batson:
"Simply defined, 'operations' is running the trains on your layout in a way to simulate prototype railroading. This simulation can take any degree of complexity, depending on the abilities and desires of the designer. The size of the layout will also affect the complexity of the operations, but it is not necessary to have a large layout to run trains like the prototype.
"Designing a layout to simulate railroad operations may not be everyone's goal, but it has worked for me, and I encourage you to at least give a serious look at it."
In my 46 active years in this hobby, I have yet to come across a more succinct and cogent summary of model railroad operations. Note that Mr. Batson does not stress a particular type of operation over another (CTC vs. TT/TO vs. TWC, etc.), nor does he stress the need for any particular car-card, tab-on-car, waybill or switch list system. He leaves these aspects to ". . .the abilities and desires of the designer."
Rather, the nugget here is found in the first sentence: ". . .running the trains on your layout in a way to simulate prototype railroading."
This is an excellent definition of model railroad operations.