Given that assumption, existing trains could also sustain something in the vicinity of 400 km/h by your own words.
Germany rebuilt and supplemented some of its railroad network for the ICE, at enormous cost (e.g. Wikipedia says the 250 km from Hannover to Berlin cost around 2.5 billion EUR, not adjusting for inflation). Of course that's nothing compared to rebuilding an entire transit network from scratch. I'm not sure any western country has even attempted to do that post-WW2, I guess China's recent investment into high speed rail is the closest modern example.
Another interesting point of comparison with existing rail networks aside from average speed is efficiency. A well utilised high speed train is more efficient than a car, but it still uses a heck of a lot of energy (with corresponding emissions, somewhere). Given the reductions in drag, I'd assume -- hope -- that a Hyperloop train will use less energy per passenger kilometer than existing trains.
Maybe the hyperloop can do sharper turns at high speed, and probably next generation of the pods or the loop allows for higher speed in general. But that the winner of the first attempts promises 396 km/h and not the +1,000 km/h we heard about before, does deflate the whole idea.
This essentially removes the requirement of flat land from the equation.
Also, remember that the hyperloop is really much more geared towards the American need: large cities are separated by much greater distances than Europe, meaning that a faster and cheaper per kilometer solution is needed than high speed rail.
https://en.wikipedia.org/wiki/High-speed_railway_line#/media...
There is a huge project upgrading existing railway tracks throughout Germany to make it possible getting from Berlin to Munich in under 4h by 2017.
So selected, in fact, that the DB is currently procuring over 300 ICEs limited to 250 km/h, and a grand total of 17 (with an option of up to two more!) that peak at 320 km/h.
I'll call myself lucky if the train manages 180 km/h on my usual routes.
They are not a replacement for those trains running fast services (with legs with 300 km/h in them).
Meanwhile, the more ubiquitous current 280 km/h models are scheduled to be replaced with a more economical 250 km/h one – and they won't even reach 200 km/h on most tracks.
ICx / ICE 4 are meant to replace the IC/EC trains first (which are slower than ICx) and later ICE 1 and 2. It seems that their slightly lower top speed shouldn't matter because of improved ac-/deceleration and relatively frequent stops (every ~70km). ICE 3 would do the 300km/h bits with longer distances. That seems like a good idea to me.
I'd like to ask on a source for your claim that ICE 3 "are scheduled to be reduced even further." The series 407 trains are just being rolled out.
Also, by definition most tracks will always be slow because there are many local / less-used tracks where such speeds simply don't make sense. Most long-distance tracks are pretty fast, and lots of upgrades are being built or in planning. I find your point misleading.
On very few select tracks – the Shinkansen could drive 500 km/h, but is still limited to 320 because of a lack of suitable high-speed tracks. Same goes for the TGV, and the Germans don't even bother with trains faster than 250 km/h.
Speaking of which, Germany sold their maglev technology to Japan and China in the first place because there was nowhere to put it at all. With Germany's winding railroad network it's impossible to reach speeds high enough to justify maglev.
400 km/h sustained, over tracks longer than 400km, would already be a massive improvement over the status quo.
Assuming, of course, the Hyperloop will be able to build a sufficient rail/tube network.