Preparing Operations

I finally knuckled down and started to create all the waybills I need to operate the trains. Of course, the idea is you complete a session, moving each car to where the waybill tells you, then turn all the cars to reveal the next spot to place the car. Since each waybill has 4 instructions, the car will not return to where it started until the 5th session.
Setting this up is harder than it first appears. Not every car goes from industry 1 to industry 2 and back to industry 1. Some of the hopper cars are captive, mean they go from the mine, to an industry, then back to the mine. Others, like a boxcar or gondola, move to a industry with a load, get unloaded then routed to the next destination empty. Or arrive empty, then ship with products. So I am taking my time setting this up.
First, I decided to set all the sidings at full capacity. This was helpful as I realized some were really a bit overcapacity. I took a car or two off a couple of sidings and eliminated the PRR Freight Station from the LAS Fabrics siding. Here is how the layout looks with the sidings "full".

PPG (left) and HJ Heinz in Irwin hace room for 2 cars each

For Uniontown, Dailey's in the foreground usually has 2 hoppers and 1 tank car; the others have 1 car each

Herminie has room for 14 cars at the mine, and 6 on the P&LE interchange

There are 3 sidings in E, McKeesport and the east most holds 4 cars

LAS Fabrics Co has room for 2 cars

May Stern Furniture has room for 2 cars

Of course, the temptation is to route more cars to fill the sidings. May Stern can actually hold 3-4 and LAS Fabrics could easily hold 3. The problem is routing the cars to the next locations with the sidings that full. With the capacity I listed under the photos, that's about 35 cars on the layout. Ideally, these 35 would move to a staging yard (except for the few going back to WCC Mine) and new cars would come onto the layout from staging to replace them. "Staging" is a loose term since I have room for 6 cars on the P&LE interchange and with the two tracks hidden behind E. McKeesport, I can represent them as one line for all points east (7 cars) and one for all points west (8 cars).
Meaning...I need to push 35 cars off the layout and replace them with 21 cars.
That's not going to work!