A straight-ish track is a baseline requirement for high-speed, to maintain top speed (220mph in the most recent generation) you need a turn radius of a few miles.

That hasn't been strictly true since the tilting APT ran commercially in 1981. It was designed for sustained 160mph on 100mph line-curves.

The patents for the tilting mechanism were bought by FIAT who debugged the technology and it's now common on UK railways in the Pendolino trains.

Shame the Pendolinos very rarely (if ever) get up to full speed as our track maintenance and rail infrastructure in general is so poor.

The UK is also one of the most expensive places in the world to travel by rail, but that's a whole different story...