OK, I think I have it just about completed. I just need to get a couple more straps of varying lengths to accommodate riding options (seat, handle bars...) for different size riders.
Here's a couple pictures of the braking system
The first shows the sliding block that the trolley hits to stop the rider. There's rubber pads on both ends to soften the impact. The rubber stops are from a horse/cow stall pad (1" thick heavy-duty rubber).
At the other end of the 40' bungee cord is a steel stake driven into the ground. Both ends are attached with carabiners.
The third picture is the "catch-all, end-all" brake block. This once is clamped hard on the cable and doesn't move. Should a rider be going so fast as to not be stopped by a certain point (about 15' from the tree at the end of the ride), the sliding block impacts this block and the rider is stopped there. In general, speed and stopping distance is based on rider weight. My son (the smallest rider) only moves the bungee about 15'. When I ride, the bungee stops me about 4-6' from that terminal block. Riders over 200# get close or may hit it.
The last picture shows the run from the starting block. If you look closely, you can see the long bungee cord on the left side of ride. In the picture, it's a black line that goes from the far left side of the picture (just in front of the wood side of the sandbox), just under that pine tree and up to the cable.