The history behind New York City’s missing subway lines

http://qz.com/549388/the-history-behind-new-york-citys-missing-subway-lines/

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At first glance, there is logic to New York City’s numbered subway lines: 1, 2, 3, 4, 5, 6, 7.

The letters, however, are a little less straightforward: A, B, C, D, E, F, G, J, L, M, N, Q, R, S, S, Z. What happened to the H? Or the O? And why in the world there are three S trains—shuttles going between Grand Central and Times Square in Manhattan, between Prospect Park and Franklin Avenue in Brooklyn, and to Rockaway Park—when there could easily be an S and, say, a K?

To understand how this came to be, we need to go back 111 years, when the subway opened in Manhattan on Oct. 27, 1904.
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