The New York subway system seems like a mess because it is three systems, that were competing more than they were cooperating, forcibly welded together. The Interborough Rapid Transit Company (the IRT) opened the first subway in Manhattan in 1904, competing with the already well-established elevated trains; the Brooklyn-Manhattan Transit Company (under its old name, the Brooklyn Rapid Transit Company) was already operating elevated trains (in Brooklyn) at that time and got into the subway game shortly afterwards; and the city-owned Independent Subway (IND) was opened in 1932 to create a public option to the private companies and to replace elevated trains with subways on more or less the same routes. The two private systems were sold to the city in 1940 and the task of merging them has been going on ever since. Since the cars on the IRT lines are narrower (same track width, of course) than the other two, the IRT lines have remained separate from (but connected by transfer stations to) the partially-merged lines of the BMT and IND.
The IND had letter designations for its routes from the start, most famously the A train. The IRT routes received numbered designations in 1948; the BMT routes received letter designations that were an extension of the IND letters in 1960. For a long time, the standard line numbers for the IRT were 1, 2, and 3 for the Seventh Avenue line; 4, 5, and 6 for the Lexington Avenue line, and 7 for the Flushing line. There was a 9 train introduced in 1989 and discontinued in 2005 that was a sibling to the 1, with the 1 and 9 running skip-stop in upper Manhattan and the Bronx. That leaves the question, where is the 8 train?
When I was little kid, the answer was clear to me: the 8 train was in the Bronx, connecting the 149th Street/Third Avenue Station on the 2 and 5 (AKA “The Hub”) with the Gun Hill Road station on the 2 and the 5. It was on the map. It also disappeared when I was eight.
The old 8 train was the last operating piece of the Third Avenue elevated. The other three Manhattan els (Second, Sixth, and Ninth Avenues) were demolished before World War II; the Manhattan portion of the Third Avenue was operating until 1955, and was torn down explicitly because it was about to be replaced by the Second Avenue subway…which partially opened in 2017. The Bronx portion of the el, part of which was built by the (believe it or not) Suburban Rapid Transit Company, was kept because there was no other rail serving that corridor. There still isn’t, and the busses added when the 8 was discontinued in 1973 don’t make up for it. You can certainly look up the 8 train on Wikipedia, but the transit authority seems to have memory-holed it.
Here’s the south end of the elevated structure near 149th and Third. You cans ee where the girders that used to go further south had been cut away:
And here’s one of the stations, looking very much like an IRT elevated station. That’s no surprise, as the IRT bought out the elevated lines around 1902.