5 Photos - Dec 30, 2014
Photo: I know Lego is anti-violence so I'll clarify that the figure is holding flamethrowers to melt snow, at least that's what my kids tell me. 

This is one of the early models and very simple. There are no adjustments on the pulleys.  The brick sticking out the side helps level (roll axis) the vehicle as it travels down the zip line.Photo: A blurry photo of this early vehicle on the cotton zip line. There was no way to adjust the pitch angle and the bottom pulley was of questionable value.

The cotton thread, while thin, was not as thin as the kite string and also not shiny and slick like the kite string. Vehicles traveled down this line slower. Lego figure with a grappling hook didn't slide at all. 

 You can see another line in the background, that's the kite string and the preferred line my kids kept going back to.Photo: This vehicle design is inspired by an Alpine gondola car. The protruding viewing platform keeps the gondola level (roll axis) when suspended by the pulleys from the zip line. Without this sidecar, the vehicle would have an extremely negative pitch.

The rear pulley is adjustable and pivots up and down to adjust the nose up / nose down (Pitch Axis) of the vehicle during descent. 

Using multiple pulleys aligned in parallel with the vehicle front/back keeps the Yaw Axis true zero. 

The pulley wheels are just lego wheels with the tires removed and they are all free-spinning. One could slow the rate of descent by adding an additional wheel that didn't rotate.

NASA has a nice simple image to further clarify Roll, Pitch and Yaw https://www.grc.nasa.gov/www/k-12/airplane/Images/rotations.gifPhoto: Side view of the gondola car highlights the adjustable rear pulley. By pivoting the pulley down the nose of the gondola rises (Pitch increases) to keep the vehicle level during its descent down the zip line.Video: bombs away. The zip line is made from Kite string. I tried other materials but kite string is kinda slick which allows the Lego vehicles to move pretty easily down the incline. You also get a nice zzzzzzzzzzzzz sound, bonus!