Redundancy

I'm in NYC for a few days, and over the weekend it got a bit confusing to take the subway because an awful lot of routes had been re-routed for the purpose of track maintenance. Confusing, but still doable -- if you followed the signs and were willing to put up with perhaps a bit of a hassle, you could get where you wanted to go.

That highlights one of the advantages an extensive rail system like the NYC Subway has over a small one like Metro in DC -- the proliferation of lines and existence of separate express and local tracks on many of them creates redundancies in many parts of the system and makes it possible to shut some sections down without causing the entire network to crash. DC, by contrast, is essentially operating with no margin for error so problems anywhere near the middle of the city spill over and create huge problems for the whole system.