Sorry, I just caught a typo in my response. The lettered lines are the ex-BMT & IND lines, and the cars are larger than the numbered lines.
The photo was taken at the Queensboro Plaza station, a very busy junction. The inbound 7 line runs beneath this outbound track; this track arrangement is duplicated on the other side of the station platform for the N line to Astoria.
The two lines are separate because of the different clearances of the cars. the numbered lines are originally IRT, and the cars are shorter and narrower than the numbered lines (BMT & IND).
A good site for NYC subway information is nycsubway.org. This site has detailed drawings of the subway track plans, including a detailed drawing of the Queensboro Plaza area.
Today's Trains Photo of the Day shows a NY Subway outbound #7 train on a single track elevated structure. Is there another inbound track out of the picture, or are there many single track sections in the subway system?
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