Question: Ive read before that if something happened to make a train break in two. The brake pipe will set the brakes in emergency for both sections of the train for two minutes. What happens to the cars that broke off after the two minutes? Will they just start rolling? Answer: No. They will not go anywhere, at least for a while. Here’s why. The key thing to understand is the air pressure in the train’s brake line is actually keeping the brakes from applying. The pressurized air in the reservoir tank under each freight car wants to push the piston and brake shoes against the wheels, but the higher pressure in the brake line prevents this from happening. This arrangement of relative pressures is controlled by the ingeniously-designed triple valve. When the brake line pressure drops below the reservoir pressure, the brakes apply. This is called a “failsafe” system—if the brake system fails, if brake line pressure is lost, the brakes apply. In normal brake applications, the locomotive engineer reduces brake line pressure by relatively small amounts, which causes the brakes to apply gradually and partially. When a train separation occurs (the train breaks in two), usually the result of a broken coupler knuckle, the train’s brake line will suddenly be open to the atmosphere and all of the air pressure will dump out, from about 90 pounds per square inch (PSI) to 0 in seconds. When this happens, the reservoirs on each car jam that piston into the brake rigging, the brake shoes slam into the wheels, and the train will come to a screeching halt (albeit over a thousand feet or more if the train is heavy and hauling butt). In theory, the air brakes should hold the separated cars in place indefinitely. However, in reality, the air pressure holding the brake shoes in place will bleed off over time, perhaps several hours, days, or even weeks (each car is independent at this point, so one car having released brakes is not a problem; its a problem when most of the cars have released their brakes). In any case, air brakes are to stop trains, not to keep them in place when stopped. That is what hand brakes are for. (Hand brakes and air brakes both activate the exact same under-carriage rigging that drives the brake shoes against the wheels; air brakes use air pressure and pistons to move the rigging, whereas the handbrakes use chains and pulleys to move the rigging.) When a train separation occurs, the crew in the cab may not necessarily know what has happened. If they cannot see a separation, all they know is the brake line is broken because it won’t recharge. Someone, usually the conductor, conditions permitting, is going to have to walk back for a look at the situation. If the conductor finds a train separation, the conductor should secure the separated block of cars just as he or she would when leaving any equipment standing on the main (setting some number of hand brakes, typically five cars, or 10 percent, whichever is greater—although this varies and is usually covered in employee timetable special instructions). So, no, the separated cars sure shouldn’t go anywhere for quite a while after the separation incident; however, air brakes are to stop trains, hand brakes are to keep them stopped. Just as soon as it is practical to do so, the separated block of cars should be secured with the appropriate number of handbrakes. Once the train is re-coupled, the hand brakes can be released. Thanks for asking. Stay safe out there! Here we go!
Posted on: Sun, 07 Sep 2014 23:49:25 +0000