A question on air brakes.

[railfan365]

My understanding of how air brakes work on trains includes that unlike road vehicles on wich the brakes will be off and go on when the driver applies pressure by way of hydraulic or mechanical mechansisms, the brakes will be held off by air pressure and go on when the engineer releases the air. And the system includes that if a hose or connections suffers damage or separation, the loss of air will stop the train. Also, it is common practice to dump the air on a train that is to be parked for a while to prevent unintended movement.

With all of that said, my question is: How do eingineers in switching maneuvers move cars without connecting air hoses?

[DutchRailnut]

Train brakes work by adding air pressure to brake cylinders. the feat is accomplished by charging air reservoirs with brake pipe.
on Freight its around 90 Lbs, on Passenger its around 110 Lbs.
The reservoir charges through a triple valve and on reduction of brakepipe the air from reservoir will apply the brakes on each car.
When leaving cars standing the air is to be dumped and hand brakes applied. cause if not dumped, the air could equalize and release the brakes.
On yard moves the air is not always cut in and movements are controlled with Independent (locomotive) brake.

[ricebrianrice]

Train air brakes are opposite from Tractor Trailer brakes.

Tractor Trailers: The brakes are engaged until air is applied, a trailer not hooked to air, has its brakes ON.

Trains: The brakes are engaged when air pressure is applied, a freight car not hooked to air, has its brakes OFF.

I work in the trailer body building industry, and this really confused me about train air brakes until I did some Google searching. There are some good diagrams our there, that help this make sense.

[10more years]

The train air brake systems work by charging the train line, storing air pressure in a resevoir mounted on each car, and then applying air to the brake cylinders by taking air out of the train line.
Cars being switched with the air hoses disconnected usually have the air bled off. Typically, when a train gets to its destination and yarded (or sets cars off in a yard), the car department "bleeds" the cars off. There's a "bleed" rod connected to the resevoir system on each car where the carman can release air pressure in the system. After the cars are bled off, they can be moved and stopped with locomotive power.

Even cars that have "air" on them will eventually lose that air through leakage unless air pressure is maintained by locomotive or alternate source, which is why operating rules dictate that crews don't rely on the airbrake system when securing cars. Hand brakes must be used to secure cars.

[railfan365]

If I understand correctly now - when a train is out on the road,the cylinder pressure is zero during movement with the brakes off, and there's pressure in the pipes. During braking, air is released from the pipes and forced into the cylinders to apply the brakes. Thanks for the replies.

[LocoCam]

If I understand correctly now - when a train is out on the road,the cylinder pressure is zero during movement with the brakes off, and there's pressure in the pipes. During braking, air is released from the pipes and forced into the cylinders to apply the brakes. Thanks for the replies.

No, When a train is moving the brake chamber has pressure on both sides or the piston, when applying the brakes air pressure in the train line (and thus ONE side of piston is REDUCED, causing brake application in the amount of the difference in pressure from one side of piston to the other. In the yard when humping/switching the air on both sides of the piston is vented, also causing the brakes to be released (again because of the pressure on both sided of the piston being equal.

Hope this helps.

[toolmaker]

This is a great discussion so I went searching for a video showing the brake system in action. I found this item to share.

The vast majority of the world's trains are equipped with braking systems which use compressed air as the force to push blocks on to wheels or pads on to discs. These systems are known as "air brakes" or "pneumatic brakes". The compressed air is transmitted along the train through a "brake pipe". Changing the level of air pressure in the pipe causes a change in the state of the brake on each vehicle. It can apply the brake, release it or hold it "on" after a partial application. The system is in widespread use throughout the world.

link to diagram http://www.railway-technical.com/air-block.gif

read more at http://www.railway-technical.com/air-brakes.shtml

[LocoCam]

This is a great discussion so I went searching for a video showing the brake system in action. I found this item to share.

The vast majority of the world's trains are equipped with braking systems which use compressed air as the force to push blocks on to wheels or pads on to discs. These systems are known as "air brakes" or "pneumatic brakes". The compressed air is transmitted along the train through a "brake pipe". Changing the level of air pressure in the pipe causes a change in the state of the brake on each vehicle. It can apply the brake, release it or hold it "on" after a partial application. The system is in widespread use throughout the world.

link to diagram http://www.railway-technical.com/air-block.gif

read more at http://www.railway-technical.com/air-brakes.shtml

Just to remember after first "charging" the airline to operating pressure (90 psi for freight / 110 psi for Pass) you REDUCE air pressuer in the airline to apply the brakes with the air you have stored in the rail cars tanks at 90/110 psi.

[railfan365]

These posts really illustrate it. Two other questions then are :

1. Do I remember that a runaway train is due to such frequent braking that the brake pipes and auxiliary tanks can't recharge fast enough for needed brake application?

2. An an EMU train, such as in the NYC subway, when the brakes are applied at a terminal stop at one end of the train and released at the other,is that accomplised by way of the brake levers having a shut down position? And in shut down, the brake system is left with the evacuated pipes being shut off from the air compressors and the atmosphere so that the valvs could be set for charging by a brake handle anywhere else in the train?

Thanks.

[LocoCam]

These posts really illustrate it. Two other questions then are :

1. Do I remember that a runaway train is due to such frequent braking that the brake pipes and auxiliary tanks can't recharge fast enough for needed brake application?

2. An an EMU train, such as in the NYC subway, when the brakes are applied at a terminal stop at one end of the train and released at the other,is that accomplised by way of the brake levers having a shut down position? And in shut down, the brake system is left with the evacuated pipes being shut off from the air compressors and the atmosphere so that the valvs could be set for charging by a brake handle anywhere else in the train?

Thanks.

1. Basically yes

2. IDK

[virtualchuck]

I'm glad this topic came up, as I have a question about air in years. Recently I have noticed in a couple of yards (one is exclusively for autoracks) that there are air hoses at the end of each siding. I assume this is used to keep the train "charged" and save time when power arrives. That being the case, wouldn't that make it easier for a cut of cars to roll away? Obviously the cars should stop as soon as they separate from the hose, but it still seems like I am missing something.

Chuck Pullen

[LocoCam]

I'm glad this topic came up, as I have a question about air in years. Recently I have noticed in a couple of yards (one is exclusively for autoracks) that there are air hoses at the end of each siding. I assume this is used to keep the train "charged" and save time when power arrives. That being the case, wouldn't that make it easier for a cut of cars to roll away? Obviously the cars should stop as soon as they separate from the hose, but it still seems like I am missing something.

Chuck Pullen

Most yards have "ground air" at least on their departure tracks, it is there so that the car dept can do a class 1 test on the cars & then leave them on air till the train crew brings the engine over & departs with the cars. If the air lines were not there and the cars were left off air after 4 hours the test would need to be repeated. Its also one of the reasons engines are left running most of the time when connected to cars. Its an FRA thing.

Hand brakes keep things from rolling away.

[virtualchuck]

Makes sense to me LocoCam, thanks!

Chuck

[GCarp]

When leaving cars standing the air is to be dumped and hand brakes applied. cause if not dumped, the air could equalize and release the brakes.

I have a question about when there is multiple cars left standing, Are the handbrakes set on all cars, just the end cars, or some combination? Would only one car set with handbrakes (say one end of the cut) hold multiple cars? Ok multiple questions...

[litz]

Each railroad will specify in its operating rules and/or timetable the number of handbrakes required.

Factors include weight and type of car, type of track, and (most importantly) the slope of tbe track.

On the BRSR, for example, we set all of the handbrakes when tying down the train.