A zip line consists of a pulley suspended on a cable, usually made of stainless steel, mounted on a slope. It is designed to enable a person propelled by gravity to travel from the top to the bottom of the inclined cable by holding on to or being attached to the freely moving pulley.
It engages gravity to assist its speed of movement. Its use is not confined to adventure sport, play or tourism.

During my visit to USA last summer to visit my son, he took the entire family for a tour of California coast line and St. Louis where we found this Zipline adventure sport. I did't have the stomach to undertake it, but my son and daughter-in-law did it a couple of times. This is one such shot which I took from my android cellphone camera OnePlus3T when my son was almost at the finishing spot. I slightly cropped the image, slightly adjusted for light and clarity and resized to FAR requirements.
Enjoy!