Actually, The Universe of Energy Ride and The Great Movie Ride technically are trackless vehicles. While there is a groove in the floor with a cable embedded in it, it does not physically pull the vehicle along the course. The cable has a distinc signal "broadcast" through which a sensor on the vehicle follows. The vehicles steer and power themeselves. The vehicle's electrical energy is stored by capacitors located within each vehicle, that recharge by contact plates in the floor at every point the ride stops along its course. On GMR this would be the wizard of oz scene, etc. and on UOE this would be the various stops in each theater.
I guess you could call Indy and Dinosaur trackless, because the track in the slot below the vehicle does nothing to dictate the direction the vehicle is traveling in. This is essentially an electrical buss bar system that transmits computer communication and power to the vehicle, but if you watch the ride as it operates, you will notice that the vehicle will be steering itself through all the turns (4 wheel steering, indidentally). This steering is totally independent of the track, and the computer must calculate every move carefully to be sure the vehicle doesn't accidentally disengage itself from the sled (whick sometimes happens - causing a ride stop). Also, you will notice, especially in the dart room section of Indy, that the vehicle does have some lateral freedom from the track, as it shimmies from side to side, but the track remains dead straight.
These systems are all really complicated, so I'm sorry if this post is confusing. I've learned all of this from general observation and talking with people who have first hand knowledge of the ride systems design.