The nether subway is a massive network of tunnels in the nether, connecting many different parts of the world. With few exceptions, it is entirely at y=12 in the nether. Some of the original parts of the subway were built to be used by minecart, and so there is track on those. Later extensions however have not had track built, and they are therefore only walkable. Using them is still much faster than traveling in the overworld.
It was originally built by suit1337, going out in just a few directions. Later when the Mushroom Kingdom coords were released publically, suit and RevStoningpot created an extension to the Mushroom Kingdom, which was at the time the furthest the subway was extended from spawn. From this extension it also connected to Damocles, but it is unknown when or by whom this extension was built.
In early 2018, Porkington and Yukar9 began expanding the subway, initially building a square grid around spawn, later extending it to other bases in the overworld, and various found paths in the nether created by other people. They also mapped their exploration, with the map being available at https://c4k3.github.io/nethersubway/ and editable by anybody.
As of September 2019, the subway is connected to