Racetrack Key is a key found in Lara's Home in Tomb Raider III.
With the Racetrack Key one is able to enter the Quadbike racetrack area located in the Garden.
The Racetrack Key is located in the Cellar (which was Lara's Treasure Vault in Tomb Raider II), in an aquarium-like pool filled with water and harmless fish. To get there, Lara has to enter the hidden part of the Attic through her library and perform a time run all the way downstairs.
It is also possible to get to the racetrack without the Racetrack Key: simply perform the Cornerbug and jump of the roof onto the hedge surrounding the racetrack. Lara will take a lot of damage, so make sure you aim for the highest part of the hedge.