A zip-line

Zip-line

A zip-line consists of a pulley suspended on a cable mounted on an incline...

 trolley is a metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 device that connects a person or a load to a cable

Cable

A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...

 and allows them to move along the cable either under their own power or by using gravity. A trolley usually consists of a metal frame with one or more shivs (wheel

Wheel

A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...

s) that the trolley runs on. The load or user is connected to the trolley via a carabiner

Carabiner

A carabiner or karabiner is a metal loop with a sprung or screwed gate that is used to quickly and reversibly connect components in safety-critical systems. The word comes from "Karabinerhaken", meaning "hook for a carbine" in German.-Use:...

 or another form of engineered link and a harness

Safety harness

A safety harness is a form of protective equipment designed to protect a person, animal, or object from injury or damage. The harness is an attachment between a stationary and non-stationary object and is usually fabricated from rope, cable or webbing and locking hardware...

. Occasionally trolleys have a handhold or handlebar

Handlebar

A handlebar is part of the steering mechanism, in lieu of a steering wheel, for vehicles that are ridden on, instead of in, such as bicycles, motorcycles, tricycles, ATVs, snowmobiles and personal watercraft. It may also be simply the handles on a device such as a zip line trolley or a floor buffer...

 although this system does not provide any fall protection and should only be used on zip lines that are low to the ground and over water.

Some zip lines are used for true transportation value while others are purely for recreation

Recreation

Recreation is an activity of leisure, leisure being discretionary time. The "need to do something for recreation" is an essential element of human biology and psychology. Recreational activities are often done for enjoyment, amusement, or pleasure and are considered to be "fun"...

 and amusement

Amusement

Amusement is the state of experiencing humorous and entertaining events or situations, and is associated with enjoyment, happiness, laughter and pleasure...

.