Quote:
Originally Posted by jmecklenborg
This might be the case for BART, since the platforms are 700 feet long - pretty much the longest rapid transit platforms in the world. By contrast, the LA light rail platforms are 250 feet long.
Any new-build subway system can build much more of its lines as cut-and-cover. When subsequent lines are built, they typically must be built deeply to pass beneath earlier lines. The next phase of the Second Ave. subway in NYC, for example, will have a very deep station at 125th St. because it will be built perpendicular to the existing existing 125th St. Lexington Ave. subway station...which is directly below Metro North's elevated station.
Similarly, the addition of elevated transit lines to cities that have expressway networks means that the lines might have to pass very high above the existing expressways.
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If you can build the platforms within the tunnel bore, then platform length isn't a huge driver of cost. That's actually a selling point, since future platform extensions are very feasible to add capacity.
But regardless of platform length, you still need two exits from an underground station by code, and when the tunnel is very deep then both exits are extremely expensive to build with multiple escalators, high-speed elevators, and complicated structural systems, plus ventilation needs increase as well.