On the RhB that used to be a regular routine, add whatever extra cars to the local passenger trains. On the express trains they would add them for overflow e.g. for skis and extra mail (Christmas rush) in the winter.
Amtrak, in the past, has run a few of their box cars to carry equipment and possibly bags. Though, I admit my knowledge in this area of Amtrak’s business is limited. I once saw an Amtrak train in Texas that had a box car lashed to the engine and then the double stackers followed. What was in it, remains a mystery. Perhaps gold or money. Maybe deceased victims. Anyway, I have also seen older passenger trains lashed up with both passenger and freight cars. D&RG was notorious for doing this as many of their old photo records would attest too.