I would only expect there to be track-sharing in terminal areas -- along the Peninsula and near Los Angeles Union Station. In those areas, I also don't expect the trains to be running at their full 300+ kph speed. Simultaneously, the local Caltrain commuter rail system should be upgraded to the same electrification standard as the HSR (probably 25KV OHL, considering current practice), and their speed brought up to the 110 mph standard, which would be "high speed rail" by benighted American standards.
Outside of the terminal areas, HSR definitely needs a dedicated right of way. Otherwise, you get the situation of the initial phase of Eurostar, with the trains having to slog their way in and out of Waterloo over the "classic" lines at significantly reduced speed. By "track sharing," I meant the same way that High Speed 1 HSR lines in the UK will include the domestic services out to Kent as well as the existing trans-channel Eurostar services.
You are of course correct that having the tracks in a shared ROW can lead to a conflict due to maintenance needs. OTOH, looking at the congested areas where the lines need to run, the only sensible place to find the necessary ROW is along the existing rail corridor. That almost certainly will lead to some land takings in some places. Alas, the Southern Pacific (and its successors) did not retain the original wide ROW intended for a four-track main line, nor did they build the remaining 2 km to a terminal in downtown San Francisco over a hundred years ago. Had they done so, things would be much different today.
In some areas along the ROW, there are light-industrial businesses (auto shops and the like; I've had my van repaired at one of them in Belmont) on year-to-year leases. Someday they will be told, "We need the right-of-way back, so we won't renew your lease next time," and they will have to find a new place to work. But all transportation improvement require some disruption. My normal office commute goes through a freeway interchange (I-880/CA-92) that is undergoing a massive multi-year rebuild/improvement. I understand some homes had to be acquired (and razed) to make room for the improvements. My father-in-law's land is bisected by a state highway that almost certainly will be twinned someday, and he'll lose more of his land when it happens. So it's not only railroads that impact landowners.
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Outside of the terminal areas, HSR definitely needs a dedicated right of way. Otherwise, you get the situation of the initial phase of Eurostar, with the trains having to slog their way in and out of Waterloo over the "classic" lines at significantly reduced speed. By "track sharing," I meant the same way that High Speed 1 HSR lines in the UK will include the domestic services out to Kent as well as the existing trans-channel Eurostar services.
You are of course correct that having the tracks in a shared ROW can lead to a conflict due to maintenance needs. OTOH, looking at the congested areas where the lines need to run, the only sensible place to find the necessary ROW is along the existing rail corridor. That almost certainly will lead to some land takings in some places. Alas, the Southern Pacific (and its successors) did not retain the original wide ROW intended for a four-track main line, nor did they build the remaining 2 km to a terminal in downtown San Francisco over a hundred years ago. Had they done so, things would be much different today.
In some areas along the ROW, there are light-industrial businesses (auto shops and the like; I've had my van repaired at one of them in Belmont) on year-to-year leases. Someday they will be told, "We need the right-of-way back, so we won't renew your lease next time," and they will have to find a new place to work. But all transportation improvement require some disruption. My normal office commute goes through a freeway interchange (I-880/CA-92) that is undergoing a massive multi-year rebuild/improvement. I understand some homes had to be acquired (and razed) to make room for the improvements. My father-in-law's land is bisected by a state highway that almost certainly will be twinned someday, and he'll lose more of his land when it happens. So it's not only railroads that impact landowners.