The Fusion Steel D Screw Locking 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 Screw 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 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.