The reason it is still carried on highways is because the highways are heavily subsidized while the railroads are not.
For example, most of the reason for expensive highway maintenance is the fatigue damage cause by trucks. Fatigue damage goes up as the cube of the weight, and the weight fees charged trucks don't remotely pay for it.
Source?
The main damage passenger cars do is erosion from snow tires.
> Estimates of pavement damage by trucks, the largest per-mile external cost of truck use, range from about 5 to 55 cents per mile depending on the weight of the truck, the number of axles over which its weight is distributed, and where it operates....
https://www.cbo.gov/sites/default/files/112th-congress-2011-...
The fuel tax is 30 cents/gallon of diesel, which for an 18-wheeler getting a typical 6 mpg is about 5 cents / mile.
https://en.wikipedia.org/wiki/Fuel_taxes_in_the_United_State...
So at the absolute highest end of that estimate you need to increase fuel taxes to $3/gallon, i.e., double the price of fuel to ~$6/gallon. That would make trucking 40% more expensive (since that's the fraction of operating costs devoted to fuel).
http://www.thetruckersreport.com/infographics/cost-of-trucki...
And a more realistic number might be closer to 20% or 30% more expensive. This would change things on the margins, but it's not clear to me that it would be transformative in pushing transport to trains.
I'd be very glad to see truck taxes/fees adjusted to better match reality. If that means less truck and more trains, all the better. Still, it's not clear to me that trains don't also get implicit subsidies (that are just less discussed because trains don't inspire as much politics), or that the positive externalities of trucks are important, or whatever.