The reason places like Tokyo, Hong Kong and Singapore have such great public transport is precisely because the rail planners work hand in hand with urban planners. Railway construction is largely financed by property sales (both residential and commercial) and when done well is hugely profitable, as the former richest man in the world, Yoshiaki Tsutsumi of the Seibu Corporation, can attest.
Retroactively building a subway line in an already densely populated area is a hugely expensive exercise, as most recently demonstrated in NYC itself with the 2nd Ave extension.
I think this is pretty commonly known, but the railroads built the US American West. It was a hugely speculative endeavor where railroad companies would buy up worthless land for cheap, hire unfavored immigrants and work them harder than anyone else would work, and then recoup costs by selling the land of the railroad towns.
Retroactively building a subway line in an already densely populated area is a hugely expensive exercise, as most recently demonstrated in NYC itself with the 2nd Ave extension.