Though I agree that NYC's number of four-track lines makes it unique, I do not think it surprising that other systems do not have many four-track lines. Given

Though I agree that NYC's number of four-track lines makes it unique, I do not think it surprising that other systems do not have many four-track lines. Given a choice between a four-track line and two two-track lines, the latter is often preferable. A piece discussing this in the context of the DC Metro: http://greatergreaterwashington.org/post/30817/metro-doesnt-have-four-tracks-thats-not-why-maintenance-is-a-problem/ http://greatergreaterwashington.org/post/30817/metro-doesnt-...