Maybe you missed it. The routes with a lot of cars are using a lot of cars because they are required to provide enough axles to carry enough current to trigger crossing gates. Meanwhile, routes that are starved for cars are sold out, and not even carrying enough passengers for the connections to other roads to get seats. So, the prices on the sold-out trains are escalated.
I don't feel I particularly miss anything there. There's still no surprises there.
Minimum train lengths due to crossing/signal reliability and/or other operational requirements, even though the passenger space isn't needed.
Maximum train lengths due to station/platform capacity, locomotive hauling capacity, equipment availability, other operational requirements, etc. so the train(s) get sold out.
The busy line is busy, the not-busy one isn't.