Trucks are much faster than train. Depending on the cost incurred in having goods be unsellable for some period of time, trucks often make sense.
Trains also aren't that much cheaper. I'm not sure why this is, but my hunch is that much of the cost in transportation is pick up from origin and drop off at destination and this essentially has to be done via truck. So, if you're shipping via train, you incur an additional pick up and drop off, since the goods need to go from the trucker's hub to the train, then from the train to the trucker's hub. That probably erases a lot of the savings that you see when you just calculate costs based on the physics of truck vs. train.
The reason trucks are cheaper is because the roads are paid for by gas taxes on cars. The weight taxes on trucks do not begin to cover the fatigue damage caused by trucks. Most of the repaving of highways is due to truck damage.
Railroads, on the other hand, pay all their own track maintenance, and even have to pay property taxes on the track. The whole system is tilted against railroads.
FWIW, I didn't say that trucks are cheaper. It's been a while since I've priced this stuff out, but, from what I recall, trains are cheaper for going cross country. Your point still stands, though, if it's correct.
Trains also aren't that much cheaper. I'm not sure why this is, but my hunch is that much of the cost in transportation is pick up from origin and drop off at destination and this essentially has to be done via truck. So, if you're shipping via train, you incur an additional pick up and drop off, since the goods need to go from the trucker's hub to the train, then from the train to the trucker's hub. That probably erases a lot of the savings that you see when you just calculate costs based on the physics of truck vs. train.