Trains are far more economical per ton-mile. The issue arises at the "last mile" if you do not happen to have both ends of your shipment adjacent to a rail line (preferably with its own siding so you don't have to deal with demurrage fees, etc.) and then have to do a truck-based leg.

Assuming you need to terminate by truck on at least one end of the shipment, it takes about 500 miles for rail's natural efficiencies to make it cost advantageous. That's a very rough rule of thumb, but it's rare to be able to make the case for rail in less than that. (Some specialized situations, like mining ore in unit trains, are exceptions.)

The reason it isn't is passenger-to-driver ratio. It's already widely amortized to have a train driver. Sure it's an easier problem, but it's just so much less of the overhead of having a train.