Rail is mostly used for cargo. Rail is about 4 times more efficient than a truck is fuel wise. Labor wise, more than an order of magnitude. A 100-unit train can carry 10,000 tons whereas a truck is closer to 20 tons.
Rail is only mostly used for cargo in the United States because there hardly is any passenger rail left. In the rest of the world rail is mostly used for passengers, and also (usually at night) used for cargo.
* the weight of freight significantly impacts the track's lifecycle, and HSR tend to be on the "light cargo" side with a train being about 10% cargo by weight
* the bulk and varying shape makes it impossible to build high-speed trains with any interesting flexibility and capability as they need to be highly profiled
* and you don't want your high-speed passenger traffic to be held up by freight.
* the cost/benefit calculation makes no sense, it's much more price-efficient to carry freight using gigantic regular-speed trainsets than using small high-speed ones, and what little freight is so time-sensitive that you need to put it on an HSR can be put there as a special case
None of the big HSR countries use it for anything resembling serious freight, the only one which came even remotely close was France which had 3.5 mail trainsets, which were wound down and ultimately retired in 2015: https://en.wikipedia.org/wiki/SNCF_TGV_La_Poste
China has some freight going on HSR nowadays, though they are mostly on slower lines (i.e., 200-250km/h, so 125-155mph IIRC) that were opened on new routes.
But yes, weight is a big deal: the LGV in France, for example, have a maximum axle loading of 17 tons. Much of the US freight network is over 35 tons, from memory.
There is relatively little economic incentive to make high speed rail work for cargo. Cargo that's large enough to go by rail tends to go for the lowest price per ton that can be found, it competes with long haul trucks, barges and oceangoing ships. It does not compete with air cargo (which tends to focus on high value and light stuff).
And many shippers will absolutely not use rail freight for a variety of reasons in the USA, including slow delivery time (can't ship produce and other perishables easily), lackluster quality of service from the rail lines (they want coal, oil, large commodity business, not smaller firms), higher chance of damage; as well as not meeting minimum qty/volume for it to be worth it.
As for speed, it's interesting to compare trucks with slow steaming.
> Slow steaming refers to the practice of operating transoceanic cargo ships, especially container ships, at significantly less than their maximum speed. An analyst at National Ports and Waterways Institute stated in 2010 that nearly all global shipping lines were using slow steaming to save money on fuel.
That is Federal but there are many exemptions, especially in the western states. For instance, Rocky Mountain doubles can go to 120,000 and certain other combinations even more.
