Assuming, of course, the Hyperloop will be able to build a sufficient rail/tube network. Given that assumption, existing trains could also sustain something in

Assuming, of course, the Hyperloop will be able to build a sufficient rail/tube network. 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.