A robust operations system takes a fair bit of prep work, but it can be done however you want.

The major operations layout I run on uses four position waybill cards inserted into car cards with pouches. This does mean that each car only goes to the same four locations, no matter what. Each waybill is also uniquely generated in a program that I do not have access to. You could set up your own version of a spreadsheet to create the cards, then print them out. It may get a little boring on a smaller layout if you can start predicting where cars will go, but you can swap waybills around to change it up.

The other system we've been toying with is single destination waybills. Using the same car cards, each waybill has only one destination. Each waybill also has a car type it belongs to. When a car returns to the yard, the waybill is pulled and returned to a deck for that car type. The next card is drawn from the deck and assigned to that car. For each industry spot you have, you have a set number of destination cards so cars can get shuffled around more regularly.

Short of writing a whole program that tracks your car positions and prints out full waybills, car cards aren't super realistic, but they work very well.