A bit of everything. Past 300 km/h, you get that little bit more drag, the downforce on the rails wears out the wheels and infrastructure a bit more, the gears, motors and bearings also take it. Larger motors and inverters also cost more.
Then there's the operational aspect - longer signal blocks, train spacing, stopping distance, potentially lower acceleration (unless you really overspec motor, the 300 -> 400 km/h zone takes its sweet arse time compared to the lower end).
To cruise at 400 km/h also requires noticeably more power. The 25 kV supply is in no way constrained but electricity itself also costs money.
Above 300 the aero starts to get a bit funky. Tunnels in particular become a massive problem and portals have to be designed carefully to prevent nasty pressure waves.
300/320/340 is already competitive enough with airlines in the spaces where HSR is competitive, so why go faster? I think there's an upper limit to the "go faster and more passengers show up" formula anyway.
Testing if i have been banned or not((You were))
What? So people can get banned for testing if they have been banned? :o
No, I'm guessing he was banned for being a shitposting idiot.
Yup. I asked him to quit doing the "necrobump" shit and explained that this is the slowest chan ever, but he kept doing it. Another admin figured that the user wasn't making any kind of redeemable contribution anyway, so adios, amigo.