Zero Gravity Cliffhanger racing sets were slot car sets produced by Tyco that had individual track pieces that allowed you to build your own unique tracks. The sets included things like flags and guardrails that would make the tracks look more realistic.
If I recall correctly, there were at least two different sets, one with a pair of red and blue Corvettes, and the Nite Glow edition where the body of the cars and pieces of the track would glow-in-the-dark.
The cars were roughly 3-4 inches long and had a small but weak magnet that kept the car on the track. You would be able to build parts of the tracks against a wall and race the cars along the walls, hence the "Zero Gravity" part of the name.