[the_hangman]

My understanding is that building subways in LA is costlier and more labor-intensive than in a lot of the cities you mentioned, but the subway system is currently undergoing a massive expansion in LA.

The main difficulties are the fault lines: Subway lines have to be meticulously planned to avoid the dozens of faults as well as other areas that are susceptible to major movement.

http://www.cccarto.com/faults/la_faults/