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The Under Subway (Japanese: アンダーのちかてつ Under Subway) is a subway line that runs between The Under and Shadow PKMN Lab in Pokémon Colosseum. It is used by Cipher to transport Shadow Pokémon from the Lab to The Under, allowing Venus to distribute them to the winners of the Under Colosseum challenge and for Miror B. to distribute them in Pyrite Town. It is initially accessed from The Under TV Station, either by walking down a spiraling staircase or by using an elevator. The staircase also provides access to a side room with a couple of item chests. The train, called the Shadow Liner (Japanese: ダークライナー Dark Liner), is operated by using the Subway Key, which Venus accidentally drops while running away from the player.
The first time the player and their partner use the Shadow Liner to access the Shadow PKMN Lab, a pair of Cipher Peons recognize them and, in order to prevent them from accessing the Lab, blow up an unsurmountable chasm into the Lab's bottommost floor. However, the player is still able to access and obtain the key required to unlock the Lab's main gate.
It is implied that The Under Subway was built before the Shadow PKMN Lab, but its original purpose and destination are unknown.
Due to The Under no longer existing in Pokémon XD: Gale of Darkness, The Under Subway is also inaccessible in said game.