I was reading a wreck thread on the CSX forum and there was some discussion of brakes on locomotives. It was pretty obvious some people here don't have much of an understanding of how things work. This is not meant as a put-down but maybe a real basic primer of brakes would be helpful - if you don't work for a railroad, and maybe don't volunteer at a museum or tourist line that operates locomotives there may not be much chance to learn. And this is not intended to be a real technical in-depth doctorate - just an intro. Also I'm talking your typical freight loco, I'll readily admit I know very little about blended braking or ECP stuff so maybe somebody else can speak to those.
Just about every locomotive has three braking methods. (not counting dynamic brakes which is a whole 'nother topic) The hand brake is strictly mechanical - by operating a brake wheel or a lever, you tighten up a chain. Through a system of levers, this forces the shoes on (usually) one axle up against the wheels. When you leave the loco unattended rules require you to apply a hand brake. This is because the air can leak off in time, or suddenly if something ruptures. The hand brake will usually be enough to prevent a unit from rolling freely, although a loco under power will usually overcome the hand brake.
The second brake is called the Independent brake. Also known as the straight air, the jam, and other terms. This is controlled by a handle on the brake stand or the desktop. Moving the lever causes air to go into the brake cylinders, pushing the pistons out. Through levers, this forces the shoes up against the wheels. If you have several units properly set up for multiple unit operation, the independent brake handle in the controlling unit will apply the engine brakes on ALL the units in the consist. The independent brake can be gradually released. Think of your car - if you are sitting at a red light with your foot hard on the brake, and you let off slightly, the car may creep a little because the brakes are still applied but not as tightly. By moving the brake handle left partway, an engineer can partly release the brakes by lessening the pressure in the brake cylinders.
Then there is the automatic brake. A full discussion would be impractical here as you could teach a college course on all the nuances and details. Very simply, air is pumped from the locos back through the brake pipe (or train line, same thing). At each car, a pipe called a Branch Pipe "T"s off the train line and goes to the car's brake valve. Air from the train line builds up pressure in the cars reservoirs. When a train is fully charged up, you will have 90 psi (or whatever is called for) in the train line, and in all the reservoirs. In actual practice small leaks in each car come into play and you will have slightly less pressure at the rear of a long train than at the front but we're trying to stay simple here so let's assume no leaks.
When the engineer wants to apply brakes he moves the automatic brake handle, which is separate from the independent brake handle. Air is let OUT of the train line and branch pipes. The brake valves on each car react to this lower pressure by allowing air to flow from the reservoir into the brake cylinder, forcing the pistons out, the levers move, and the shoes go up against the wheels. If a train breaks in two, or the train line is opened up by a derailment, or the engineer moves the handle all the way to emergency position, full pressure from the reservoir is directed towards the piston and the brakes come on hard.
Passenger brakes can in some cases be gradually or partially released like the independent brake, but freight car brakes can't - once they are applied, they must be released by re-charging the air system.
When the automatic brake is applied, the locomotive brakes will apply also, just as if you had used the independent brake handle. Sometimes this is OK, but for good train handling it is often desired to keep them off. In that case, the engineer then uses the independent brake handle - instead of moving it left or right, he will push it DOWN. This releases the loco brakes while having no effect on the braking on the cars. This is called "bailing off" the brakes.
In that CSX forum on the wreck someone was asking how a crew could switch cars without air. If the air is manually released (or "bled") from each freight car, the cars will roll freely. The engineer can only control the train with the independent brake because the train line is not connected. But in this way cars can be "kicked" or humped, rolling freely until either gravity or other cars cause them to stop.
I'm sorry if this has been long but entire books have been written about brakes and I'm just trying to give some very basics. There are a lot of other factors which make it more complicated but this is a start. And for those who want to learn more about basic switching, switches, coupling, signals and other stuff there is a book called "The Basic Training Manual For Brakemen and Switchmen" or something like that. It is published by Simmons Boardman, has been updated a few times, and is used by many railroads as an introduction for new hires. Those of you who WANT to learn more about how things really are, but can't actually hire out might want to buy a copy. Sometimes well-meaning but ignorant fans ask questions here "Why did the train do so-and-so" or How could (something) have happened. There are also various air brake manuals but I'm not sure where the average fan can buy them.
Those of us who work on the railroads must remember that many fans ask what seem like dumb questions because they just don't know how things work and that's because they have not had the chance. But we all had to learn those things too. And the fans can help themselves understand more, and ask more informed questions, by trying to understand how and why things work as they do.
Just about every locomotive has three braking methods. (not counting dynamic brakes which is a whole 'nother topic) The hand brake is strictly mechanical - by operating a brake wheel or a lever, you tighten up a chain. Through a system of levers, this forces the shoes on (usually) one axle up against the wheels. When you leave the loco unattended rules require you to apply a hand brake. This is because the air can leak off in time, or suddenly if something ruptures. The hand brake will usually be enough to prevent a unit from rolling freely, although a loco under power will usually overcome the hand brake.
The second brake is called the Independent brake. Also known as the straight air, the jam, and other terms. This is controlled by a handle on the brake stand or the desktop. Moving the lever causes air to go into the brake cylinders, pushing the pistons out. Through levers, this forces the shoes up against the wheels. If you have several units properly set up for multiple unit operation, the independent brake handle in the controlling unit will apply the engine brakes on ALL the units in the consist. The independent brake can be gradually released. Think of your car - if you are sitting at a red light with your foot hard on the brake, and you let off slightly, the car may creep a little because the brakes are still applied but not as tightly. By moving the brake handle left partway, an engineer can partly release the brakes by lessening the pressure in the brake cylinders.
Then there is the automatic brake. A full discussion would be impractical here as you could teach a college course on all the nuances and details. Very simply, air is pumped from the locos back through the brake pipe (or train line, same thing). At each car, a pipe called a Branch Pipe "T"s off the train line and goes to the car's brake valve. Air from the train line builds up pressure in the cars reservoirs. When a train is fully charged up, you will have 90 psi (or whatever is called for) in the train line, and in all the reservoirs. In actual practice small leaks in each car come into play and you will have slightly less pressure at the rear of a long train than at the front but we're trying to stay simple here so let's assume no leaks.
When the engineer wants to apply brakes he moves the automatic brake handle, which is separate from the independent brake handle. Air is let OUT of the train line and branch pipes. The brake valves on each car react to this lower pressure by allowing air to flow from the reservoir into the brake cylinder, forcing the pistons out, the levers move, and the shoes go up against the wheels. If a train breaks in two, or the train line is opened up by a derailment, or the engineer moves the handle all the way to emergency position, full pressure from the reservoir is directed towards the piston and the brakes come on hard.
Passenger brakes can in some cases be gradually or partially released like the independent brake, but freight car brakes can't - once they are applied, they must be released by re-charging the air system.
When the automatic brake is applied, the locomotive brakes will apply also, just as if you had used the independent brake handle. Sometimes this is OK, but for good train handling it is often desired to keep them off. In that case, the engineer then uses the independent brake handle - instead of moving it left or right, he will push it DOWN. This releases the loco brakes while having no effect on the braking on the cars. This is called "bailing off" the brakes.
In that CSX forum on the wreck someone was asking how a crew could switch cars without air. If the air is manually released (or "bled") from each freight car, the cars will roll freely. The engineer can only control the train with the independent brake because the train line is not connected. But in this way cars can be "kicked" or humped, rolling freely until either gravity or other cars cause them to stop.
I'm sorry if this has been long but entire books have been written about brakes and I'm just trying to give some very basics. There are a lot of other factors which make it more complicated but this is a start. And for those who want to learn more about basic switching, switches, coupling, signals and other stuff there is a book called "The Basic Training Manual For Brakemen and Switchmen" or something like that. It is published by Simmons Boardman, has been updated a few times, and is used by many railroads as an introduction for new hires. Those of you who WANT to learn more about how things really are, but can't actually hire out might want to buy a copy. Sometimes well-meaning but ignorant fans ask questions here "Why did the train do so-and-so" or How could (something) have happened. There are also various air brake manuals but I'm not sure where the average fan can buy them.
Those of us who work on the railroads must remember that many fans ask what seem like dumb questions because they just don't know how things work and that's because they have not had the chance. But we all had to learn those things too. And the fans can help themselves understand more, and ask more informed questions, by trying to understand how and why things work as they do.
