French TGV climbs 3.5%, German ICE climbs 4%. That simplifies a lot of the route planning. Rail bridges are surprisingly simple and cheap (open area, known loads, no salt) and tunnels are easier the simpler the rock.
I'd say it's no harder than any of the other miracles that we're building on the regular.
To put it another way, Europeans haven't built high speed rail from London to Baku for similar reasons. The tree lined cloisters where political theory makes all nation states equal is not a good model of physical reality.Swiss rail hasn't connected Bern to Baghdad.
The Swiss did vote for and consequently built things like the Gotthard Base Tunnel.
The Chinese are also building a lot of rail infrastructure and are of a similar size to the US. But then they have higher population density. But it's probably more a question of political priorities than physical realities.
The US geography makes high speed rail at the Federal level economically infeasible. Air transport reaches everyone including Alaska and Hawaii and Puerto Rico.
Now air transport can reach everyone, but is also especially climate unfriendly. Maybe in the future there will be electric or hydrogen airplanes, but if there is an option to use rail or plane mass transit, rail will always have lower energy demands. But historically the US has low energy (and fuel) taxes, and that will favor the planes. EU on the other hand is now talking about taxing aircraft fuels and if that happens and the taxes are sufficiently high, trains will be more attractive, even if currently travelling long distances by rail in the EU is a huge PITA.
I’d venture that American rail could have looked similar if the North American continent was made up of dozens of countries instead of just one, forcing at least one of those countries to use the Rockies more comprehensively.
Once you get to the western mountains, there are only four rail grades that are suitable for freight. The old Northern through Montana near Glacier National Park; the original Union/Central Pacific route along the Platte and over the Sierras around Donner Pass; the ATSF route across the Colorado Plateau south of the Grand Canyon at Flagstaff and over the Sierras at Tehechapi via the loop, and the all weather grade through the Gadsden Purchase along the Mexico border.
The reason there are so few routes is simply geography. North America's mountains run north-south and there is more than a thousand miles of them between Denver and the coast. And since the west is dry much of what is west of the first line of mountains is desert as well as mountains.
Those are the grades that work for freight to the west coast. Notably, none serve much population between the Mississippi River and the Pacific. West of the Mississippi the US population density is low. The densest state California has about 1/5 the density of the Netherlands. The second most dense, Arizona is 1/20 as dense.