This article by a railway engineer goes into the limits of train speeds and why speeds of most commercial trains seem to have stagnated at the 350 kmph mark for decades.
Trains with diesels engines are slower than electric ones; the world speed record for a diesel train was set at 238 kmph in 1987 and remains unbroken. Electric trains however can theoretically hit around 600 kmph.
So why don't most commercial electric trains go at 600 kmph? One problem is it is difficult and expensive to engineer trains that go at such speeds – aerodynamic drag increases as the square of speed, so the faster you want to go the more aerodynamic your train needs to be to operate at that speed.
The second major limitation is energy consumption, especially when trains need to stop frequently at stations:
It is interesting to observe how a non-stop train with a target top speed of 300 km/h achieves a total energy consumption which matches that of another train with three intermediate stops (skip-stop service) and a top speed of 250 km/h. Conversely, if we set the top speed at 300 km/h, the train performing skip-stop service requires almost 30% more energy than non-stop traffic.