400 km/h seems to be practical. China started building trains that can go that fast. They must already have lines that support that speed, cause upgrading lines

400 km/h seems to be practical. China started building trains that can go that fast. They must already have lines that support that speed, cause upgrading lines is the huge expense. There is less advantage to increasing speed. Is it worth the cost of maglev Shinkansen to save 30 minutes? How long would 400 km/h rail take instead? Maglevs have disadvantage that need dedicated route, while high speed rail can take existing rail into cities.