Although what you say is correct - there are some things you can do to
'mix it up' that lessen how many off layout cars you need/need to swap
in and out ...
Don't let your crews always take the same jobs.
Use 4 part way bills.
Swap way bills between cars of the same type between sessions or
even between trips out of staging (fiddle the way bills in staging).
Add in the specific 'demand for and response to' empties - most let
the waybills act as the empty supply ... if you have a yard clerk job
you can model the actual empty supply part of the equation. You
can even add in that the clerk has to pay attention to the car
forwarding rules and the captive service cars.
Add in "clean out track" operations (yeah, I know, this is difficult to
do with existing trackage). Or maybe your boxes/drawers of 'extra'
cars can model that - first the car goes to staging, then it gets its
waybill pulled, then it goes to the cleanout drawer, then to the
available drawer, then back to staging, then a waybill assigned
and actually in a train for the next op? Perhaps the cars in
staging get "clean out waybills"?
Modernize an industry (swap in a different structure?) and change
its demand from 40' cars to 50 footers.
Recruit from a larger crew base - so that not everyone is always
the same. One layout I know puts out a 'call for operators' once
a month - his crew size is 18! - he has about 25-30 guys on his
call list. First 18 responders get jobs, the rest wait till next
month. He often fills his crew in under 48 hours and has filled it
as little as 4 hours. Yes, his crew calls go out in emails and the
responses are emails. He puts out his call "about 10 days before