> All you need is a platform level with the flatcar.

Do you?

You can't drive a truck sideways (from the side of a parked train); you can't drive a truck across multiple rail platforms (from the end of a parked train); and you can't efficiently split up the train in individual cars, bring them to a rail-end where they can be driven to/from and reconnect them, that would be much more expensive in terms of machinery (locomotive) and operator use than a crane; train cars can be reshuffled but it adds significant time and effort compared to just lifting a container from a truck onto a train, which is how it's usually done. You would also need specialty (longer) flatcars to fit not only the length of the container but the truck as well.

There used to be TOFC (trailer on flat car) cars that had little flaps that you could lower so that you could drive between cars. They don't have those any more, for the most part. They found they could make them less heavy with a "spine" kind of design, which you can't drive across.

Also most intermodals these days use well cars which allow containers to be double stacked.

https://upload.wikimedia.org/wikipedia/commons/e/e5/DTTX_724...

> you can't drive a truck across multiple rail platforms (from the end of a parked train)

This is how we loaded our vehicles onto train when I was in the US Army. Everything from small vehicles to large trucks. There were removable pieces connecting the cars so they could be driven over.

Think of it like parallel parking. Not much room is needed, and the flatcar + the gap between cars can be longer than the truck by just enough to make it work.