The issue isn't so much the distance from the train, so much as once you take it off the train, you split it up into much smaller loads....with different owners and destinations, and those loads probably change hands, and are pretty hard to put -back- onto trains.
It sounds like we need an RFC for automated packet-switched transport of physical goods. Surely the existing algorithms used for physical logistics and IP network routing could be used to create a highly-reliable automatic package delivery system.
The closest I can think of is Personal Rapid Transport[1],
which is a light-rail/monorail network of individually powered (small; 3-person or so) vehicles. Various schemes for routing have been proposed, many of which are conceptually similar to packet-routing of the carriages.
