The first sets, brought out in 1965 by Ideal, were called 'torture tracks', for example, Dearborn Test Track, Alcan Highway Torture Track, etc. There was a variety of cars from the beginning. These were powered by AA batteries. The early cars had the option to free run on a floor, and their wheels could be placed in a variety of positions, allowing for circles, straight line racing, etc. Also, a pin could be installed on any car. This pin would engage the groove that ran down the center of each track.
A very entertaining slot car set was the Motorific Line made by Ideal. These came in a variety of sets, separate cars, trucks, boats (called Boaterifics) and accessories. What was different about Motorific was that each car or truck was powered by two AA batteries. There was no contacts in the track. Hence, track assembly was VERY easy, although you could not control the speed of the car.
There were really three different types of sets:
These were the first, originally designed as 'test tracks', with a variety of tests that are performed on the car, as if it was a large proving ground. Tests included horsepower test, steering test, spring test, and crash test.
These used the same track as Motorific, but they added a timer and additional items to each set. The idea was to compete with others to see who could get through the course in the shortest amount of time, or in a rally scenario, who could come closest to the handicap time.
In about 1967, Ideal released the Action Highway line. These sets featured the wider yellow track. The old cars could run on this too, but the new track would not connect to the old track. Reminds me of how Tyco used to change tracks and create compatibility problems. However, the yellow tracks were also a breeze to connect, they just lay together and simply lock down. Although the fit is sloppy, the cars pass along fine, and there is no road contacts to worry about.
Trucks came in each Action Highway Set.
Source: http://www.prestonmarketing.com/toys/motorific/motorific.html (offline).