The Subway Glitch is a glitch in Grand Theft Auto III, that allows the player to walk on top of the subway tunnel.
This glitch can be performed at the subway in Shoreside Vale at the airport. When the train arrives to the platform, take the very last door of the train. Enter the train and then get out again. You will automatically get teleported above the platform, into the Blue Hell and you will see the tunnel. A word of caution, sometimes the tunnel will get invisible and you risk walking off the edges and get teleported back to the streets.