Air brakes are air brakes. It doesn't matter what gauge track the equipment runs on. WABCO made different sized equipment for different sized cars. They made K-1 triples for smaller, 1900 era equipment. As cars grew bigger and heavier, K-2 came along. K-2 is simply a bigger system with bigger brake cylinders, reservoirs, etc. to get better braking effort on heavier cars. The narrow gauge cars all used K-1.
By the time AB brakes came along, cars pretty much all weighed the same and different systems were not needed. Changes in braking forces could be handled by changing the leverage on the body of the cars. This made life a bunch simpler when all freight cars had the same basic brake valves.
The exhaust port in a retainer valve gets smaller as one turns the valve to retain higher pressures. Release position pretty much lets the air out the brake cylinders as fast as it goes out through the triple valve. In low pressure position, it exhausts at a much slower rate. Later retainers with a "slow direct" position exhaust at the same rate as low pressure. The difference being a retainer with low pressure setting will hold 10 lbs of air pressure in the brake cylinder while, a slow direct position will eventually release the brakes.
The exhaust port in the High Pressure position is smaller yet. A full service brake application develops 64lbs of cylinder pressure, When the brakes release, it takes several minutes for the pressure in the brake cylinder to drop to 20 lbs, which where the "High Pressure" setting is supposed to stop exhausting air.
This is the important part, and the part I had a difficult time driving home to the guys I taught air brake classes to.....
It isn't the 10 lbs or 20 lbs of air retained in the brake cylinders that is saving your life coming down Cumbres or La Veta. It is the restriction in the cylinder exhaust that makes it possible.
For every pound of trainline reduction you make, you get 2.6 lbs of brake cylinder pressure. So, a 10 lb set gives you 26 lbs in the brake cylinder. If you then release the brakes, and the retainer is set on high pressure. eventually the cylinder pressure will reduce to 20 lbs and hold there. 20 lbs of brake cylinder pressure barely wakes up the brakes on a 130 ton car of Perlite. It does nothing to help the braking effort.
But, now for the magic part. Take a 10 lb set, get 26 lbs of cylinder pressure. Release the brakes. As the brake pipe and car reservoirs recharge, the air is slowly releasing from the cylinders. It might reduce the cylinder pressure to 22 lbs. Then, make another 10 lb set. That puts 26 lbs air back into the brake cylinder, ON TOP of the 22 lbs that is still there, giving you 48 lbs of cylinder pressure. Now were getting somewhere. Release it again, charge the system up, the brake cylinder pressure eases back a few lbs to 44 lbs. Make another 10 lb set, putting 26 more lbs into the cylinders on top of the 44 lbs already in there. We now have a whopping 70 lbs of cylinder pressure. Now that 130 ton car of perlite is going to slow down on that 3% grade.
When I was running over La Veta, that is how we crested the hill. It took 3 sets and releases to feel there was something back there. I made a 4th set, and held on to it and throttled up to drag the train over the top (at 5mph). If it pulled too easy, I'd back off, release the brakes, let it charge up again and give it another 10 lb set. That would usually make things settle down and drag hard. Over the top with the air set. Slowly back the throttle off keeping the speed around 5mph. Finally, you got to idle. 10 seconds later you went into dynamic braking, bunching the slack. Soon you were in about notch 3-4 in braking and the train was beginning to slow. Release the air. The train would continue to slow. Then stabilize. Reset the air. By the time the brake set is seen on the rear end, the train will have begun to accelerate. Then the set will take hold, the train will again begin to slow. Release the brakes again. Repeat a hundred or more times going down the hill.
There ya have it. Short Cycle Braking with retainers.
The big thing to remember is it is not the fact that the retainer "retains" air that saves your butt. It's the itty bitty hole the air exhaust through that does that job.
Take a short break, have a smoke and drink of water. The exam will begin in 10 minutes. no cheating. Your life depends on it........
