Intro or Intro Sequence is a track from early development created by Probe Entertainment (Acclaim's producer) which would be used to Re-Volt's introduction. It looks like a dark room with a spotlight in the center lighting a wooden reflective floor. The track's default data folder name is 'intro'.
The original idea was to create an animated introduction to Acclaim and Probe Entertainment using Probe UFO and the Probe Model. The introduction could be described with the UFO car perforating the Acclaim's logo, then landing on the floor while drawing the Probe's arc with a rainbow path. Finally, the Probe logo falls inside the formed circle. 
Theoretically, the introduction and the track were removed from development since Probe Entertainment was acquired by Acclaim in 1999, then became Acclaim Studios London. The current intro sequence is formed by three pictures known as Intro 1 (legal), 2 (Acclaim with game website link) and 3 (Acclaim Studios London).
The track was first found in Developer Version of Re-Volt, then later restored to 1.2 and used as a third introduction. The track is hidden by the game and is unraceable, requiring series of modifications to became visible on the track selection screen (except by running the game with -dev command). When the player gets access to the track, the cars are placed in odd positions, and everything around is pretty dark. 
The new introduction from Re-Volt 1.2 consists in Probe UFO arriving the wooden floor and drawing a rainbow circle where the Probe logo falls afterwards and then dragged out by two cars (RC Bandit and Evil Weasel).