ICE test train reaches speeds of up to 405.0 km/h

This article communicates what this is about very poorly, so I'm not surprised a lot of people are asking "what is so special about this train?".

The answer is: Nothing. Many ICE trains have the capability to go that fast* - and some already surpassed these speeds on test tracks decades ago. It's really nothing special to make a train go these speeds.

What this test was supposed to show is that the real track (not a test track) between Erfurt and Leipzig/Halle can now support trains going that fast. Having compatible tracks is the real challenge (and cost sink) for high speed transport, not the trains themselves. Creating high speed track that is safe and usable in year-round conditions while being affordable to build and maintain is surprisingly hard.

* ICE-3s reached up to 368 km/h in tests, though ICE-4s are designed for more economical speeds in the 200-300 range and currently limited to 265km/h in software for safe operation.

Also, the passenger miles per unit of energy drops geometrically, as speed increase linearly. Most of the loss is aerodynamic, so you either need a hyperloop tunnel or wings to take you into the literal stratosphere, to avoid high fuel burn at high speeds, so even if you could run the train at 400 km/hr along the whole track, it would be unlikely that any operator would do so, on a recurring basis.