The Upstate Tunnel is primarily a two-lane road tunnel, which presumably connects Liberty City with Upstate Liberty.
Grand Theft Auto III
In Grand Theft Auto III, the tunnel is closed for construction. There are piles of dirt and signs at both entrances of the tunnel. This is supposed to be the limit of the game's map, as there an invisible wall to prevent the player from passing.
Grand Theft Auto: Liberty City Stories
In Grand Theft Auto: Liberty City Stories, the tunnel is accessible. The walls of the tunnel are rocky and it has lighting. In the middle of the tunnel there is a small road that is closed.