Unlike a railway train, trucks operating in fleets assemble and disassemble easily, without stopping, when their routes converge or diverge. They also can operate with a number of other vehicles between them.
Rails are a more efficient interface to wheels, but also more demanding, and less flexible. 10-lane freeways are widespread, but most railways have two pairs of rails at best, and one pair in many places. You can't effectively use them without strict scheduling.
Rails are a more efficient interface to wheels, but also more demanding, and less flexible. 10-lane freeways are widespread, but most railways have two pairs of rails at best, and one pair in many places. You can't effectively use them without strict scheduling.