Shinkansen in Japan regurarly does 320 km/h and 360 km/h is being tested, with the main issue being running noise and tunnel boom.
These speeds are really not theoretical and we regurarly made use of them during travels in Japan. For example back in 2017 we traveled 1100 km from Beppu to Tokyo in about 8 hours. This contained a normal speed express train segment from Beppu to Fukuoka (1:30 h, max speed 130 km/h) and two train changes.
Also a maglev based line between Tokyo & Nagoya is under construction, which will be 80% underground with a running speed of ~500 km/h.
These speeds are really not theoretical and we regurarly made use of them during travels in Japan. For example back in 2017 we traveled 1100 km from Beppu to Tokyo in about 8 hours. This contained a normal speed express train segment from Beppu to Fukuoka (1:30 h, max speed 130 km/h) and two train changes.
Also a maglev based line between Tokyo & Nagoya is under construction, which will be 80% underground with a running speed of ~500 km/h.