The basic Equipment of each freight wagon

The Basic Equipment

Each freight wagon has an air tank (reservoir) on it including open top wagon, hopper wagon ,flat wagon,tank wagon,etc , This reservoir (often called the auxiliary reservoir or brake reservoir) is charged with compressed air from the locomotive's air compressor thru the train line brake pipe. You can see the air hoses between the cars and they go Kapowssssh when you uncouple them. After the initial charging of the reservoirs, the brakes can be set (applied) by REDUCING the pressure in the brake pipe. Compressed air from each car's reservoir pushes on the brake cylinder piston to apply the brakes on the car. In the rare event that you have no air in the reservoir then you've got no brakes and you've got a run away.

Now if you are not technically oriented or don't understand the meaning of psi (pounds per square inch air pressure) you may as well give up right here and be satisfied with the above description. If you understand compressed air and have a high boredom threshold then read on.

OK. How are the brakes controlled? That is the job of the so-called "triple valve" on each car. Basically this valve compares brake pipe air pressure and car auxiliary reservoir air pressure. If the brake pipe pressure is HIGHER than the reservoir pressure, the triple valve moves to the RELEASE position. In this position it vents any brake CYLINDER air to atmosphere thus releasing the brakes. It also connects the BRAKE PIPE to the RESERVOIR so brake pipe air pressure can begin recharging the reservoir. This is the situation you are in when you are CHARGING the brake system sitting in the yard waiting for a brake test ("pumping up the air").

So now the locomotive's diesel engine is turning an air compressor that is pumping air thru my brake valve into the brake pipe of the China railway wagon , back along the train and thru the triple valves of each car into the reservoirs. This takes a lot of air. It takes anywhere from 15 minutes to an hour to charge a train depending on its length and how leaky the air hose couplings are. On the railroad I work for the standard brake pipe pressure is 90 psi.

Brake Application

Once the cars' reservoirs are charged to the same pressure as the brake pipe (90 psi), the brakes are ready to be used, either on the road or for an air brake test.  Now suppose I want to set the air brakes. What I do is move the brake valve handle from the RELEASE & CHARGE position to the APPLICATION position. This disconnects the loco's air compressor from the brake pipe and opens a small hole to vent brake pipe air pressure to atmosphere. This venting of brake pipe air causes the brake pipe pressure to drop slowly.

On each railway vehicle, the triple valve is "watching" the brake pipe pressure and the reservoir pressure. Remember when the system was charged, both reservoir and brake pipe were at 90 psi. Now with the brake pipe pressure dropping, the triple valve senses that the brake pipe is LOWER than the reservoir. This is a signal to the triple valve on that car to move to the APPLY position. In the apply position it connects the RESERVOIR air pressure to the BRAKE CYLINDER. This air pressure pushes the brake cylinder piston out and applies the brakes.

Meanwhile, up in the cab, I watch my gauges and when I get the brake pipe pressure lowered to where I want it, I put my brake valve in neutral or LAP.  Lap simply seals the brake pipe, letting no more air out and not letting any air compressor air into it.

Let's say I "made a 10 pound set".  This means I reduced the brake pipe air pressure from 90 psi to 80 psi then "lapped" the brake valve.  Remember the triple valve on the car was watching the brake pipe air pressure and, as soon as it dropped below reservoir pressure, it moved to the APPLY position and allowed reservoir air to flow into the brake cylinder.  This flow of air from the car reservoir to the cylinder will of course lower the pressure in the car reservoir. Remember that the triple valve always watches the pressure in the brake pipe and the reservoir.  It allows air to flow from the reservoir into the brake cylinder UNTIL THE RESERVOIR PRESSURE LOWERS TO THAT OF THE BRAKE PIPE PRESSURE.  Now once again the reservoir and brake pipe are equal pressure so the triple valve returns to its own LAP position. Both now have 80 psi.

But now, all that air that flowed from the reservoir to the cylinder has applied the brakes on that car.  The volume of the reservoir is about 2.5 times the volume of the brake cylinder.  So to lower the reservoir 10 psi, from 90 to 80, enough air flowed from the reservoir into the cylinder that it put 25 psi (2.5 times 10 psi reduction = 25 psi) in the brake cylinder.  Simple isn't it?

I now have the choice of leaving the brakes applied to control speed or stop, or I can make another reduction to get heavier braking, or I can release them. Let's say I want to slow down faster than I am so I want more braking. I move my brake valve to the application position and lower the brake pipe another 5 psi from 80 to 75 psi. The triple valves on the cars sense once again that the brake pipe (75psi) is LOWER than the reservoir (80psi). So they move to the APPLY position and allow more reservoir air to flow into the cylinder until the reservoir is the same pressure as the brake pipe (75psi). The brake cylinder pressure goes up and so the braking effort goes up.

Because of the 2.5 ratio of reservoir volume to cylinder volume this 5 psi reduction results in (2.5 times 5psi =) 12.5 psi more braking pressure. This is on top of the 25 psi already there for a total of 37.5 psi brake cylinder pressure. Notice that if this air brake system has been fully charged, it is "fail safe". That is, anytime the brake pipe air reduces, the brakes apply. Thus if a train comes uncoupled or an air hose bursts, the brakes apply fully, automatically. The amount of braking relies on the amount the system is charged however.

Brake Release

When I no longer need the brakes I can release them. This is done by moving my brake valve to the RELEASE & CHARGE position. As before, this simply connects the locomotive air compressors to the brake pipe and pumps air back thru the brake pipe raising its pressure back to 90 psi. The cars' triple valves sense that the brake pipe (90psi) is HIGHER than the reservoir (75psi) and move to RELEASE position. This connects the brake cylinder to the atmosphere, releasing the air pressure in the cylinder and thus releasing the brakes. It also connects the brake pipe to the reservoir to begin recharging the reservoir from the brake pipe. Notice there is NO GRADUAL release; the release is a complete release.