Comment by cowsandmilk

It is much more expensive to build another line at that point due to the increase in density of the city (and sometimes new requirements of stations to meet building codes). Nearly every high volume transit system out there has choke points that are extremely expensive or impossible to fix by building another line. The tunnel between VA and DC on WMATA’s Orange, Silver, and Blue lines is a 20 year project. Tracks for LIRR to go to Grand Central took 60 years from proposal to opening. It is viewed as nearly impossible to build a line parallel to the MBTA underground green line, a system that has short two car trains and 4 branches above ground on the Boston side, so very easily could use a parallel line.