Abstract: A conveyor system having an upper power track with a propelling device for selectively engaging and propelling trolleys along a lower free track from which they are suspended by carrier arms. The propelling device has a pivotally mounted hook engageable with an opening in a carrier arms of the trolleys. The hook is mounted in an arresting device which has an inverted U-shaped recess into which the ends of the carrier arms can be selectively inserted. The distance between the power track and the free track is shorter along inclined sections of the conveyor system than along level sections. This shortened distance between the tracks brings the carrier arm closer to the arresting device in a manner so that the end of the carrier arm is inserted into the U-shaped recess. When the end of the carrier arm is inserted into the U-shaped recess, the arresting device is able to control movement of a trolley along inclined portions of the conveyor system.