GBC_2 has series of devices which balls can take. The rough ball route of the series is signified by the red and green markings in the first picture. Both series start and end at the Ball Freeway, a tilted construction on which the balls roll. It is circled in yellow. It is tilted towards the bottom side of the picture, towards the gate. The gate tilts left and right, as seen in the second and third pictures, to let balls onto either the first or second series of devices.
This is the first series of devices. First, the gate is opened to the right and one ball falls into the Ball Elevator, a construction that can go up and down. The Ball Elevator rises up, and pushes the ball off onto a series of ramps (picture one and two). The ball completes the ramps and falls into the Mine Cart (picture three). (The battery in the mine cart is to increase weight over its drive wheels, hopefully increasing traction.) The Mine Cart travels down guide rails to the right of the machine until it reaches the end (picture four). The Mine Cart flicks the ball into the first ball lift, where it lifted back into the top of the Ball Freeway.
This ball series of devices moves balls through them much more quickly then the first series does. In this series, first, the balls go down the powered treads, and fall off the end onto a small ramp and into the catapult. There is a light sensor at the high end of the ramp, intended to detect balls as they go into the catapult (picture one). When it detects one, the treads stop, it waits, then the catapult flings the ball into the second ball lift (picture two). That ball lift was designed to bring balls up (picture three) to the device that they roll back to the Ball Freeway on (picture four).
Here is a video of GBC_2. The first part of the video demonstrates a ball going through the first series of devices. The second part of the video displays balls going through the second series of devices. Partway through the video, I restarted the program for the NXT operating most of the second series' devices so it would work. I may have done that multiple times during the video. Thanks for looking!