Garage door openers that use an operator rail turn a chain or belt. This piece connects to a trolley, which is also called a carriage. The trolley pulls the opener arm to lift the garage door, and pushes the arm to close the door.
A piece of rope typically hangs down from the trolley. This is used to disengage and reengage the opener. When the opener is disengaged, pressing the wall button or the button on your remote will run the opener and turn the belt or chain. It will not, however, open or closed your door.
You can sort the trolleys by manufacturer, or you can browse other garage door parts using the links below.