One of the major advantages of trucks over trains is precisely that - they're not trains. Making them trains would take away all the flexibility they currently have. If you're taking coal from the mine to the power station then you're probably already using a train. Anything less predictable needs individual trucks.
First, having 2-5 trucks in an automatic convoy is still ENORMOUSLY different than a multi-locomotive train with 10+ or even 100+ wagons on fixed rails.
Second, even if you equalized their "sizes", a three-truck convoy is NOT magically equivalent to a three-wagon train, because they are running on fundamentally different infrastructures.
