ExCon90 wrote:Without searching for that thread, I would assume it would have to do with the longer stopping distance needed by a HS train vs. a commuter train, which raises the question of why anyone would want to mix the two on the same tracks in the first place.
It does sound impractical, but in the USA, we do have an example where the same line has both commuter trains and trains that are the closest thing the USA has to "high-speed": AMTRAK's NEC between Philadelphia and NYC.
The question isn't whether you'd want to mix high-speed and low-speed trains on the same track at the same time, but rather whether you'd want to sometimes use a particular track for high-speed trains and for low-speed trains at another time. After all, a track can only run trains in one direction at a time, but even on lines where a given track runs in one direction most of the time, the tracks are usually signalled for traffic in either direction.
I think this is likely to be a common situation for HSR in the USA. Acquiring completely separate rights-of-way is expensive, prohibitively so in metropolitan areas, and "metropolitan areas" tend to cover a lot of land due to sprawl, so I think that large chunks of any HSR route are likely to be existing routes, upgraded for higher speed and with an extra track where necessary.