LAYOUT DESIGN FOR OPERATION
I backed the HO-gauge Alco road switcher up and coupled to the (anachronistic) string of freight cars in Boston Yard. I leaned over with my uncoupling wand to separate the 1949 stock car, with roof walk, open door, sawdust on the floor, and steer gazing out the door, from the rest of the string--all 55-foot 3-bay center-flow covered hoppers. The father of a young spectator said to his son, "Watch what he's doing; he's fixing it." I uncoupled the cars and moved forward. The father said, "What is he going to do with it?" I moved the stock car past the switch, threw the switch, and backed the car onto a string of cars with roof walks. I attached a caboose and engine, and the way freight was ready to move the cars out around to the stock pen and other industries along the line.
A requirement for a layout with operation is that it have places. Switching modules are great, but they aren't layouts. What is a place? A place is a place for a train to stop and, for freights, to do some work. Examples of places are stations, industry siding(s) and yards, junctions, and division points.
What makes a place? Ideally, places are separated by a nice long scenic stretch of mainline or branch line. However, this is not an operation requirement! For a credible layout, though, separation of some kind is required. What are other kinds of separation? I can think of mountains, tunnels, aisles, rivers, and vertical separation. Two places can be on the same "board" right next to each other provided a train must travel from one place around a loop, uphill or downhill, to the other place.
But the real point is that, for operation, places should be planned for in the original design of the layout!
THE DIVISION POINT
A place on a railroad where there is lots of operation is the division point. Trains must stop and change crews, possibly engines, possibly take on coal and water for the steam engine, set out or pick up cars, or even re-classify the whole train. If you have any room on your railroad, model a division point!
Another place on a railroad where you can make lots of operation is where a branch line meets a mainline. The branch line may or may not be its own railroad, or may or may not have its own engine(s) and caboose(s). A good place for a branch to join the main is at a division point!
If the branch is its own railroad, the main road will set off and pick up cars for the branch via interchange tracks or an interchange yard. The branch may have trackage rights on the main road or vice versa. And of course if the branch is its own railroad, it needs engine facilities and railroad offices, which might be located at the junction.
Once you have loaded or emptied a car at one of the industries on your layout, where will the car go? It goes either: to another industry on your layout, or off your layout altogether. The latter requires hidden layover tracks, a hidden fiddle yard, and/or hidden interchange tracks. My "waybills" send empty cars to the interchange, and usually send loaded cars to the interchange.
The interchange is switched "at night". The yard engine pulls from the yard the cars for the interchange, parks them, pulls the cars from the interchange tracks to the yard, and runs the parked cars to the interchange.
Once the cars are in the hidden, off-layout track(s), the waybills are updated. All this adds operation and adds purpose to moving the cars around.
Before you throw tracks on a 4x8 piece of plywood, consider that you will find model railroad life more interesting if you plan for operation. You can pick particular places to model and make them places on your layout. Then you can route cars from place to place, with trains to move the cars. Plan how to separate your places. Consider a division point for one place. Plan hidden trackage for cars to come from or go to when you have loaded them, and to come from or go to when emptied.