All European railway operators are legally required to offer this, by the way: it's an open market, so (provided there is physical space) they have to allow anyone to run their own train. Normally this means freight trains, but it also means companies like FlixTrain can attempt to compete with the large national train operators - and of course it allows for one-off charters.

The only downside is that preference is given to regularly scheduled services, and the remaining space is first-come-first-serve, so on the busier routes there's a decent chance you'll have to take a large detour instead, or sit in a siding waiting for a while.