[iggldiggl]

> A high speed train carries a lot of momentum, as does a slow one carrying a huge load for that matter, and it can take quite some distance to bring that to an orderly⁰ stop, noticeably more than a single signal separation distance in high density areas.

While dissipating the kinetic energy resulting from high speeds and/or high weights can be a noticeable design constraint, too, especially on longer downhill gradients, the more immediate limiting factor is simply the fact that the coefficient of friction for steel wheels on steel rails (0.1 to 0.4) is quite low when compared to that of rubber tyres on asphalt or concrete (0.4 to 1.0).

(Which is goes hand in hand with the significantly lower rolling resistance enjoyed by railways, but it does make reliably achieving higher brake deceleration rates somewhat more difficult. Though even if you could do away with that constraint, the next issue is that passenger comfort and safety would prevent much higher deceleration rates, anyway.)