I'm surprised I can't find how fast/high-speed/accelerating ski lifts work at HowStuffWorks. I based this completely on my father's and my observations, but I think this is about right. Wheel A goes into track 1 and wheels B go into track 2. Track 3 pulls wheel C down and to the left (from the perspective of the people sitting in the lift), pulling the left side of the clamp off the track. Wheels 4 all go at different speeds, accelerating D. Note that the axle that wheel A is on rotates.
After getting home where there was an Internet connection, I found the video below showing the inside of a high-speed ski lift terminal.