Wire rope for ziplines
Galvanized Aircraft Cable is the best type of cable for a zipline. It is strong and flexible and reasonably priced. The best size for ziplines under 100 feet is between 3/16 inch a 1/4 inch. Increase in diameter as length increases. A 5/16 inch diameter works well for lengths up to 500 feet and reasonable rider weight (under 250 pounds). Aircraft cable is a type of wire rope, made with a spiral of wire strands around a central wire.
Watch out for coated cable (don't get it). Coatings produce more friction and heat. I have personally heard of the plastic coating slitting and peeled off preventing the Trolley from running smoothly.
Using a cable winch will make tensioning so much easier.