As I always say in class - there are no "dumb questions". (Sometimes there are some "pretty stupid questions - smile - but there are no "dumb questions".)
While the suggestion to use the www.NYCsubway.Org
website is a good one, one has to know the website exists before there is the need to answer a question. There are several books about the subways, also.
The 9th Avenue station in question was the BRT transfer point between the 5th Avenue Elevated Line and the 4th Avenue subway line, and the West-end and Culver elevated lines to/from Coney Island. With the building of the original IND subway line and the then D train terminating at Church Avenue - it was designed that the IND would take over the Culver Line all of the way to Coney Island. The remaining section between 9th Avenue and Ditmas Avenue operated as a one-track shuttle until its removal.