We don’t need self-driving trucks for this. This is already how intermodal shipping works: a driver picks up a container and takes it to a rail yard, it gets loaded onto a train, and then when it gets near enough to where it’s going, another driver takes it from the rail yard for final delivery.

The reasons for not doing intermodal are the same as not being able to do self-driving trucks: oversize loads, document-controlled shipments (haz-mats, for example), and mixed deliveries along a route. Almost everything else could be put on a train, but it would take some infrastructure upgrades and likely unattainable levels of cooperation from shippers.