Like pretty much everything else, it's an optimization problem rather than a physical limit.
So running a train at 350kph is more expensive than 300kph, both in per-distance and pre-unit time terms. But if you can run more services that way then sufficient demand might make it economical. Also, if it's too slow, people may choose flying instead.
Maglev can go even faster but those have never been made economical, really. It's much more complicated and expensive.
It's a bit like how commercial planes have actually gotten slower. 747s used to fly closer to Mach 0.9. Now most commercial planes fly at around Mach 0.8. There are physical problems flying between Mach 0.8 and 1.2 but sometimes that doesn't matter so the best private planes top out at about Mach 0.93. Even then they rarely fly that fast.
So running a train at 350kph is more expensive than 300kph, both in per-distance and pre-unit time terms. But if you can run more services that way then sufficient demand might make it economical. Also, if it's too slow, people may choose flying instead.
Maglev can go even faster but those have never been made economical, really. It's much more complicated and expensive.
It's a bit like how commercial planes have actually gotten slower. 747s used to fly closer to Mach 0.9. Now most commercial planes fly at around Mach 0.8. There are physical problems flying between Mach 0.8 and 1.2 but sometimes that doesn't matter so the best private planes top out at about Mach 0.93. Even then they rarely fly that fast.