I have fairly extensive mileage for separate HO and O27 and O gauge layouts. My preferred method of operation is to run a variety of different trains sequentially. On the HO scale layout I have different meeting points and junctions and layovers. With the 3-rail O-gauge I typically run one train and park it, then run another train etc, sometimes doing "hopscotch" with the parking places. This type of operation I rationalize as a mix of "dispatcher operation" and "railfan viewing" with a procession of different trains getting over the railroad.
For medium and larger size layouts, I like to have a substantial visible yard area where a variety of different trains are on display and ready to run on the layout.
I also have some smaller simple one-train layouts where I just make a few circuits with the one train. Those layouts achieve a sense of small community orientation where you watch the usual daily train go by. I like the variety of different operations, large and small.
For my O27 layout I expanded the yard from four tracks to six, later seven tracks, with cut-down switches. It handles an assortment of prewar and postwar trains. Small loop-runner layouts can be fun with a variety of different trains available to run from a yard.