>>13411151That's not the entire story. The train brakes are indeed activated by a drop in pressure in the brake pipe. To release the brakes, the brake pipe must be pumped up to its full pressure, 90psi for North American freight trains. This actually fills an air reservoir on each car, and it is that air in those reservoirs--not the brake pipe--that is used to physically actuate the brakes. If the engineer cycles the air too much (sets and releases without enough time between each application to fully recharge to 90psi), you will need heavier and heavier brake pipe reductions to accomplish the desired amount of brake force, and eventually all the air in those reservoirs will be depleted and the train's brakes will be rendered inoperable until the entire system is pumped up and all the reservoirs on all the cars are refilled. Depending on train length and outside temperature, it can take as little as a few minutes or as long as hours to pump the brake system back up. It can and does happen, although it is rare.
That's a simplified version, as each car actually has two air reservoirs, one for service (normal) applications, and one just for emergencies (that is only used when the air in the brake pipe is dumped to atmospheric pressure). It's more complicated to explain, but the same thing happens, all the air in both reservoirs gets depleted, leaving no air pressure left to apply the train brakes.
