Why do they roll with no air,or it seems!

Prior to the cars going over the hump, each and every car has the air brake system COMPLETELY bled off, i.e. no air pressure remaining in the system.

Hot Water- Doesn't the system need air to release the brakes, or the other way around?

thanks for your response.

Well now, sounds like you REALLY need to improve on your "basic railfan knowledge".

Railroad air brakes work by the failsafe method.

1) The entire car or train of cars, is charged through the brake pipe air line, from the locomotive. during the charging of the train, each car receives compressed air into its two-compartment main reservoir. When complete, the entire train is at a nominal 90psi.

2) When the Engineer wants to apply the brakes, he makes a 10psi reduction, for example, to the brake pipe train line. Once the brake valve on EACH CAR senses the 10psi drop in brake pipe pressure, the valves move their internal spool and take 10psi out of HALF of the main air reservoir and directs THAT 10psi air pressure to the brake cylinders on EACH CAR. Thus, the brakes on EVERY CAR in the train apply.

3) Should the Engineer then desire to release the train brakes, he places his brake valve in the lead cab into the "release" position, and the locomotives pump the train line brake pipe back up to 90psi. Again the brake valve on EVERY CAR senses this increase in train line pressure, moves its spool to A) release the air pressure in the brake cylinders to atmosphere and B) charge the main reservior back up to the original 90psi.

4) Should ANY break in the train line occur, the train line air pressure drops to zero, and every single brake valve on every car, immediately directs the ENTIRE pressure contents of BOTH HALVES of the main reservoirs into each and every brake cylinder. Thus the train is then in "emergency".

So, you can see if there is no air pressure in the car, there are no brakes on the car, except for the hand operated "parking brake".

It's also possible to pump up the train air to release the brakes, turn the brake pipe angle cocks to closed at both ends of a car before uncoupling it, then the brakes will stay off until the brake pipe leaks down enough to set the brakes. 

It's also possible to set up the brakes on a cut of cars, turn the brake pipe angle cocks to closed at each end of the cut, and the brakes will stay applied (for an uncertain time) until the air leaks down out of the entire system.

Both actions are referred to as "bottling the air" and are both against the rules. Definitely a good way to get yourself fired, if caught!

Truthfully, I don't see the way the brakes operate as totally fail-safe.  The fact that with no air pressure in the system they're off seems to imply that a leak in the air system and that car has no brakes.

This is just a random comment, but I've wondered more than once why they were done the way they are.

GRJ,

http://www.railroad.net/articl.../airbrakes/index.php

ChipR

A couple of points about "leaks in the air system":

1) Prior to departing any terminal, a leakage test is done by the Engineer, which consists of cutting out the maintaining feature of the locomotive's brake valve, and timing for 60 seconds, how much leakage the train has (by simply watching the brake pipe air gauge). If leakage is excessive, he must report to the car inspectors prior to departing.

2) Any leakage that may occur in the system while enroute, the Engineer will notice on his brake pipe flow meeter, and above a certain indication must stop and have the Conductor walk the train for an inspection. 

Well, on the previous post, that link suggests that the air brakes are not perfect.

A problem with the design of the standard air brake is that it is possible to use up the air in the auxiliary reservoir more quickly than the brake pipe can recharge it. Many runaways have resulted from overuse of the air brake so that no auxiliary reservoir air is available for the much needed last application.

THAT is due to OPERATOR ERROR, gunrunner!

My point exactly!  Operators make a lot of errors, it's the most frequent cause of crashes in trains, planes, and automobiles!

That is a shortcoming of a one-hose air brake system that was originally developed in the late 1800's. Before dynamic braking became common, it was often necessary to manually "set retainers" on individual cars of trains travelling down long downgrades. Retainers literally retained air in the brake cylinders to keep the brakes applied while the brake pipe was recharged, which would otherwise release the brakes.

Well, nothing is totally without risk of failure, even the redundant systems on cars and planes.  Then there's ships...  no brakes at all.

But seeing we don't have freight cars regularly wandering around the country unattended or pushing locomotives into the oceans, the Westinghouse air brake system seems to work pretty well.

Rusty

I now understand the line, "It was on that grade that he lost his air brake..." in "Wreck of the Old 97".

quote:

Truthfully, I don't see the way the brakes operate as totally fail-safe.

Over the years the term "Fail Safe" has lost its true meaning. As in the statement above where it leads one to wrongly think in the term of "Totally Foolproof".

Actually, what "Fail Safe" means is that, in the case of a train's air brake system, when continuity between the source of the air (engine) and the rear car fails (air hose separation), it fails to the safe side. Meaning the brakes APPLY!

Hot Water's explanation really has nothing to do with with Gunrunner's concern as I read it.

Gunrunner's post is concerned with all of the air leaking off of the system, resulting in 0lbs. of air pressure in the trainline, aux. & emergency reservoirs. If such was the case, the air brake would not work.

Hot Water's explanation has to do with the leakage of trainline pressure. This is an old LAW put into effect well before the application of the pressure maintaining feature because when the engineer made a brake application, he didn't want the trainline to leak down any more and create more braking force, slowing the train down or even stopping.

As discussed earlier, one does not want to apply and release the train brakes too often or it eventually runs out of enough air in the auxiliary reservoirs of the cars to safely control the train and hence the need to use retainers.

The Pressure Maintaining feature will take care of a leak that happens enroute of about 26psi in the case of the 26L brake stand. As the law and Hot Water says, you cannot leave the terminal with more than a 5psi leak in the trainline.

New rules allow brake tests to be made using only the "Air Flow Meter", making the need to cut out the pressure maintaining feature unnecessary.

To answer the OP's original question, when switching cars, it is common for ALL the air to be bled out of the brake system, precisely so the cars WILL roll freely, as they do coming off a hump. Hump yards are typically designed to have a "bowl" profile, lower in the middle than on the ends. Cars without air and without hand brakes applied will not roll out of such a yard. They will tend to roll to the middle of the yard.

It is possible to switch with air - we used to do it on the Ohio Central all the time. It makes it difficult to kick cars, but we rarely needed to do any kicking, so we switched with air.

As for closing both angle cocks and "bottling the air" when cutting a car away from a train, we used to do that once in a while too, in special circumstances. Until it was made illegal. Now that's a good way to get fired.

Ace, the service portion piston works on the difference in pressure between the brake pipe and the auxiliary reservoir to graduate auxiliary air into the brake cylinder.  The piston in the emergency portion works on rate of change.  If the brake pipe pressure gets to low, say 35 psi, there is not enough pressure difference between the brake pipe and atmosphere to cause the emergency piston to move to the emergency position.  Thus when the brake pipe is worked to a pressure to low to get an emergency application, there is still air in the emergency reservoir, it is just not accessible.

As for the hump operation, an inbound train cuts its power off, a carman walks the track to bleed the air off all the cars in the track.

 The cars are shoved up the hump by the hump engines, (at the Alton & Southern big double hump, the humpers are pairs of remote controlled UP SD40's), the cars, with no air, are let go by the pin puller at the top of the hump according to his switch list. All the infomation on each car in the cut being humped was fed into the hump computer. The order, the wieght, the destination, etc. When the car is cut free, the computer has lined the turnouts into the proper track and the computer applies the "retarders" with enough pressure to control the speed of the car as it rolls down the hump.

The computer knows how many cars are already in the track and basis its speed on that, first car into an empty track will not have much retarder applied so it will roll a long way. A "skate?, at the end of the track will stop the car from rolling out. the computer will slow the cars more as the track starts to fill so coupling will be made at a safe speed not to cause damage , usually 4 MPH or less.

After a track is full, a puller, (A&S uses pairs of remote controlled switch engines), to couple the track and pull the track or multiple tracks to the outbound track where roadpower is added, the air pressure pumped up, the brakes "set" and the carman inspect the entire train to ensure that all cars are safe to depart and that the brake pipe pressure "leakage" or air flow method, is within the parameters. 

Forgive my windbag on brakes, which has little to do with this thread:

Years ago, up into the 1980's the Missouri Pacific had an air brake car, (ex passenger car) that toured the system and required engineers to attend as part of the rules classes. It was really a good class. It had the various locomotive brake valves and a number of car brake valves. When an application was "set" on the locomotive automatic brake valve, you could watch the brakes "set" on each car brake cylinder and they all had gauges so you could see what was happening.

Train air brakes "set from the front to back, so it may take a number of seconds before the rear car senses the brake pipe reduction and applies it's brakes.

With the advent of distributed power and computer controlled air brake valves on the locomotives, a DP unit in the middle or rear of the train will get the signal from the lead controlling unit computer to help draw down the brake pipe much quicker from the rear forward, appling the train brakes in a more uniform fasion. The DP units also will recharge the brake pipe at a much faster rate which will also release the train brakes much more uniformly and help prevent pull-aparts, (used to be more common, an engineer applies more horsepower to pull the train before the brakes on the rear portion on the train release all the way).

There are a number of rules today as to setting out cars individually or in blocks. Hand brake are tested to ensure that a car or cars will not roll. You may hear on a scanner, release brakes for a securement test. then a drag test to hear if the brakes on the wheels are screeching, all to make sure the hand brakes are properly applied and that enough are applied to hold the cars left behind. Now another rule, when making a single car setout, the above applies, but after yoru engine is cut away, you must wait 1 minute, then apply the handbrake somemore. (I guess the thinking here is after the emergency valve applies its air to the piston, there may be some extra tork to be applied to the hand brake?).

Anyway, we get tested or watch all the time by management to see that these rules are followed. The air brake rules are mandated both the company and the Federal Railroad Adminastration who also send agents in the feild to ensure the rules are complied with.

Dan

Thinking about the difference between truck brakes and train brakes and the "failsafe".

As has been posted, when there is no air on a train car, there is no brake applied. A hand brake must be used. A train with a bad brake pipe on the road halfway back can still move if they bleed the air off the cars behind the bad pipe and move to where the bad car can be set out.

A large truck with air brakes has two purposes, air to release a big spring that prevents movement, parking brake,  and air to apply the brakes through the brake pedal.

If the truck loses air pressure, the spring brake will apply stopping the truck. The truck cannot be moved until a source of air is applied to release the spring brake. 

In an emergency, a mechanic can crawl underneath the trailer or truck and turn a large screw which will compress the sring brake allowing the truck to move with no brakes.

Two different ways to use air to work brakes. Which one is failsafe?

Runaways: Both have happend to trucks and trains. Train of old because an engineer may have ****ed his air away, (to many applications and releases before the brake pipe could recharge the reservoirs.), coming down a hill to a point where they will not apply anymore.

I would guess that runnaway trucks are more of a cause of brakes shoes getting to hot to be effective anymore.

dan   

quote:

A train with a bad brake pipe on the road halfway back can still move if they bleed the air off the cars behind the bad pipe and move to where the bad car can be set out.

While this is true, it is also something that is unsafe and something you DO NOT want to do. The reason being, if the cars behind come uncoupled for any reason, then you have yourself a runaway! *Exception would be a single car that would have damage from, say a broken drawhead that needed to be set out on line of road.

Most NS units (and other roads) today have a runaround hose stored in the nose (or somewhere else) of the unit so that if any part of trainline pipe on a car does break, that car's air brakes can be bled off, the hose attached to the car ahead and the car behind so that pressure can be restored to the rear of the train. It can then proceed to the next terminal.

Something like that can happen to automobile brakes - and power steering.

When a motor is running, the brakes and power steering are "assisted."

If a motor stalls (or is turned off), pressure in both systems falls to zero in a few seconds. Brakes and power steering still work but they require a lot more effort.