Here is a VERY SIMPLIFIED description of how North American freight train brakes work and answers to other frequently asked questions. Basically there is a reservoir (air tank) on each car which is charged with nominally 90 psi of compressed air. This air comes from air compressors on the locomotive and is sent to every car on the train through the train's brake pipe. Once the reservoirs are all charged with air the engineer can set the brakes on the entire train by bleeding air out of this brake pipe. He does this with his brake valve in the loco cab. This REDUCTION of air pressure in the brake pipe causes a valve on each car to connect that car's reservoir air to the brake cylinder on that car. Thus the brakes are applied. To release the brakes the engineer moves his valve to the release position which once again sends compressed air back thru the train. The increase in pressure in the brake pipe causes the valve on each car to vent the air in the brake cylinder to the atmosphere. A spring in the brake cylinder of each car causes the brakes to move away from the wheels. Note that with this arrangement the brakes apply whenever the air pressure in the brake pipe goes DOWN! This means that if the train breaks into two pieces or comes uncoupled, the brakes will apply fully, automatically since all of the brake pipe air pressure will be vented to the atmosphere by the now broken brake pipe. This is why they are called automatic air brakes. But also note that until the reservoirs on each car are initially charged with air from the locomotive the train has NO BRAKES. This is because the air pressure that goes into the brake cylinder to force the brakes against the wheels comes from the reservoir on each car. This reservoir must be initially charged before it has air to supply to the brake cylinder. Again keep in mind that this is a simplification but it gives you the basics. There is nothing new about this system. It was invented by George Westinghouse in the late 1800s and has been in use ever since with minor improvements. You can "graduate" the brakes on, that is control the braking force in the "more brake" direction, but the release is full release only. You can not graduate the release like you can in an automobile by easing up on the brake pedal. This is one of many things that make running a train different from driving a car or truck. The engineer must not set too much brake or he will stop short. Too little brake and he can set some more if there is time, if not he is fired or dead! The cars I've been talking about have the air brake cylinder/piston mounted on their frames. Usually hanging beneath the carbody, or in the case of hopper cars on the end of the car. Some cars have the air brake cylinders/pistons mounted inside the car's trucks. These are very hard to see if you don't know what to look for. All the cars I've been taking about are freight cars, passenger cars are a little different so keep this in mind while Amtrakking. Passenger cars do have graduated release brakes and some have "anti-skid" but no freight cars that I am aware of have either. Some freight cars do have a load/empty sensor so brake cylinder pressures can be higher when loaded and will be limited when empty to prevent sliding the wheels. Locos have two air brake systems. One that operates just like the train brakes and another that operates more like an automobile's brakes. Faster acting and graduate on, graduate off. The locos brakes are usually used only for switching, "light engine" moves, or holding an already stopped train on level or slight gradient track. This system is operated by additional air brake hoses connected between locomotive units. Most locos have a 3rd braking system, dynamic brakes. which essentially rewires the traction motors to act as generators. The electricity thus generated is thrown away as heat in locomotive resistance grids. The force needed to generate this power comes from the rolling train which is retarded as a result. How does the engineer control the trailing locomotives? Trailing locomotives are controlled by the engineer in the lead loco. The trailing locos automatically do whatever the lead loco does. This is controlled via a multiwire jumper cable (usually 27 wires) that is connected between each unit. This cable carries commands to make the loco go forward or reverse, commands it to operate in the throttle position the engineer wants, and controls other things on the locos such as dynamic braking and even backup headlights. Won't small locomotives be pulled or pushed along by larger locos if they are combined in the same consist? There is NO problem with multiple unit locos of differing HP operating together whether they are 1000 HP or 3000 HP or 4000 HP. Each loco will do its share. Suppose a man is pulling on a rope that is tied to a heavy wagon. The wagon is too heavy for the man to move. His 12 year old son comes along and begins pulling on the same rope with his father and the wagon moves. The son is not nearly so strong as the father but the combination works. Each is pulling as hard as they can even though they are not equal. It does not matter whether the son is ahead of or behind his father on the rope. Al.