CO2 race cars are far more than toys -- they're a case study in multi-discipline engineering. In fact, many high-level universities' engineering programs have annual competitions to see who can build a CO2 race car the fastest.
CO2 cars use jet thrust to propel them through the air. A needle punctures a CO2 cartridge's covering, whereupon high-velocity carbon dioxide shoots out and propels the car forward.
CO2 cars come in three basic varieties: novelty or "replica" cars, dragsters with exposed wheels and hollowed-out "shell" cars with completely enclosed wheels.
The more aerodynamic "shell" cars have an advantage over longer race courses since they're not as subject to deceleration-inducing drag as the often-lighter rail cars. Rail cars accelerate faster but can't maintain speed as well, so they will typically have the advantage on shorter race tracks.
CO2 cars rely entirely on jet thrust, so anything that increases rolling resistance slow them down. The fastest CO2 racers use carefully aligned and very narrow wheels with precision wheel bearings to reduce rolling resistance and maintain alignment at all times.