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. I was on a main-line passenger train recently that did stop pretty hard¹ – it is a slightly disconcerting experience, feeling that rate of deceleration knowing how much force is involved.
Though as you say, a lot more of this is automated these days, with signallers and drivers prioritising efficiency more because the modern safety systems take a lot of that cognitive load of knowing what is going on further in front away from them.
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[0] minimum stopping distance is often not a pleasant stopping distance!
[1] on that section we could have been doing up to 90mph I think, though we weren't going that fast as just before the stop we were accelerating, the train came to a rest smoothly but in an unusually short time/distance
> 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.)
Also before any automated breaking is considered, there is advanced warning of potential upcoming obstructions, so the driver can begin reducing speed early: https://safety.networkrail.co.uk/jargon-buster/four-aspect-s...
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. I was on a main-line passenger train recently that did stop pretty hard¹ – it is a slightly disconcerting experience, feeling that rate of deceleration knowing how much force is involved.
Though as you say, a lot more of this is automated these days, with signallers and drivers prioritising efficiency more because the modern safety systems take a lot of that cognitive load of knowing what is going on further in front away from them.
--
[0] minimum stopping distance is often not a pleasant stopping distance!
[1] on that section we could have been doing up to 90mph I think, though we weren't going that fast as just before the stop we were accelerating, the train came to a rest smoothly but in an unusually short time/distance