I missed this first time around, but the Times has a piece on the end of the 9 train. The 1 and 9 offer so-called skip-stop service, in which each train leap-frogs past alternating stations. As the Times describes, it’s common on 9-train stops to see three 1 trains in a row pass your station while you wait for the next 9.
What killed the 9? Ironically, suggests the Times, the gentrification of West Harlem. The riders most inconvenienced by skip-stop service tend to be lower-income Blacks and Hispanics, but now that higher-income Whites are coming into the area and buying up homes, they don’t want to stand around and wait for trains to carry them downtown.
What the article doesn’t mention, however, is that the 1 and 9 aren’t the only skip-stop trains in the system. The J and Z run skip-stop along the Broadway-Brooklyn line.