Once upon a time...
There was a large city. So large, that trains ran in the sky. While this helped keep the streets clear to a degree, it blocked the sun. So the logical place was to put them underground. Come the decade that roared, and Lindy was the toast of the world, plans were made. But before a shovel could dig, the big crash came down. Oh the dream of this underground railway, was never forgotten, but it was delayed. It was ready to go again, then the day of imfamy occurred, and it was stalled again. In 1950, a bond proposal was passed, a single test train of new cars that were to run there were delivered to try out, but alas, the rest of the system was in disaray, so the bond money when to fix the existing network, no construction at all, the lone train set, would run for 30 years or so, and be regulated to various lines. Come the 1960s, oh this underground railroad, was again the talk of the town, this time, it was a bit different, money was allocated, some sections were actually built, but it stopped in the early 1970s due to a financial crisis. So these four sections of tunnel sat unused, unconnected to anything, until a couple of years ago. The whole route cannot be built at once as it is cost prohibitive, however, its been broken into 4 phases, of which the first one is now underway. The first two phases are likely to be completed, the second two, not so much.

I give you the story of the second avenue subway.
(Subway Riding the rails underground, Fri 4 Dec 2009, 1:55, Reply)