The Fusion Steel D Quick Lock Carabiner is perfect for a trolley back-up.   It can be used as a regular carabiner or used as a back-up because of its super sturdy steel design.  

Use the Steel D Auto Lock Carabiner as an over-the-trolley back-up to prevent injury in case of zip line trolley failure. The all-steel design ensures that if the carabiner runs along the zip line cable, it will not be damaged and will maintain integrity for years.  Works great used with the Contego Lanyard with Backup or by using two lanyards that tie into the harness.

Not only do you get the Carabiner, but you get the very best customer service in all the land.

If you're not happy, send it back for a full refund within one year.