Question related to train speed, passenger safety and control...

[lstone19]

What I don't understand is how a freight car left standing can become a runaway, and how cars can be humped if the brakes are applied when they're not pressurized.

How train brakes work is widely misunderstood. Air does not directly keep the brakes released. Rather, a loss of air in the brake pipe causes pressurized air already in the reservoir on each car to flow from the reservoir to the brake cylinder.

The longer explanation is starting from a train completely empty of air in the brake pipe and the reservoirs on each car, the brake pipe first acts as an air supply charging the reservoir on each car to the brake pipe pressure. Once fully charged (takes a while, particularly for a long freight train), the train is ready to move. At that point, the brake pipe is acting primarily as a control signal. Reduce the brake pipe pressure 10 pounds and the reservoir on each car also reduces 10 pounds by sending that air to the brake cylinder (due to size difference, there's a 2.5 multiplier so a 10 pound brake pipe reduction puts 25 pounds in the cylinder). If the engineer then releases the brakes, the brake pipe starts charging back to the full pressure. As soon as the pressure starts rising, the control valve on each car vents the brake cylinder and the brakes release. Eventually, every thing comes back to full pressure and things are ready to repeat.

It takes time to fully recharge after brakes are released. Repeatedly apply and release and you eventually run out of air. That's why long freight trains rarely make running releases - once you take air, you stop and then wait for the brakes to fully recharge before moving again.

Also, you can't keep taking reductions. Once the air pressure in the reservoir on each car equals the brake cylinder pressure, that's all the braking you can get (except for emergency brakes). If you brake pipe pressure is 70 psi, a 20 pound reduction brings the brake pipe to 50, 20 pounds goes from the reservoir to the cylinder putting the reservoir at 50 psi as well, and the brake cylinder, thanks to the 2.5 multiplier, is also at 50 pounds. That's full service and that's all you can get.

Back to what I said about needing to wait to recharge, if I take a 20 pound reduction, release, and then immediately take another 20 pounds, because things never got significantly above 50, all I can get is 36 pounds in the cylinder (5/7 of 50 - the first application was 5/7 of 70 which is 50). My second application is not as effective as the first.

Leave cars standing without a locomotive and eventually all the air leaks away. That's why handbrakes need to be applied to standing cars.

FWIW, while I am not a licensed locomotive engineer, I am a qualified passenger conductor and interurban operator (same brake system) at the Illinois Railway Museum where I am fully qualified on air brake operation.

 

[BlackDiamond]

How train brakes work is widely misunderstood. Air does not directly keep the brakes released. Rather, a loss of air in the brake pipe causes pressurized air already in the reservoir on each car to flow from the reservoir to the brake cylinder.

The longer explanation is starting from a train completely empty of air in the brake pipe and the reservoirs on each car, the brake pipe first acts as an air supply charging the reservoir on each car to the brake pipe pressure. Once fully charged (takes a while, particularly for a long freight train), the train is ready to move. At that point, the brake pipe is acting primarily as a control signal. Reduce the brake pipe pressure 10 pounds and the reservoir on each car also reduces 10 pounds by sending that air to the brake cylinder (due to size difference, there's a 2.5 multiplier so a 10 pound brake pipe reduction puts 25 pounds in the cylinder). If the engineer then releases the brakes, the brake pipe starts charging back to the full pressure. As soon as the pressure starts rising, the control valve on each car vents the brake cylinder and the brakes release. Eventually, every thing comes back to full pressure and things are ready to repeat.

It takes time to fully recharge after brakes are released. Repeatedly apply and release and you eventually run out of air. That's why long freight trains rarely make running releases - once you take air, you stop and then wait for the brakes to fully recharge before moving again.

Also, you can't keep taking reductions. Once the air pressure in the reservoir on each car equals the brake cylinder pressure, that's all the braking you can get (except for emergency brakes). If you brake pipe pressure is 70 psi, a 20 pound reduction brings the brake pipe to 50, 20 pounds goes from the reservoir to the cylinder putting the reservoir at 50 psi as well, and the brake cylinder, thanks to the 2.5 multiplier, is also at 50 pounds. That's full service and that's all you can get.

Back to what I said about needing to wait to recharge, if I take a 20 pound reduction, release, and then immediately take another 20 pounds, because things never got significantly above 50, all I can get is 36 pounds in the cylinder (5/7 of 50 - the first application was 5/7 of 70 which is 50). My second application is not as effective as the first.

Leave cars standing without a locomotive and eventually all the air leaks away. That's why handbrakes need to be applied to standing cars.

FWIW, while I am not a licensed locomotive engineer, I am a qualified passenger conductor and interurban operator (same brake system) at the Illinois Railway Museum where I am fully qualified on air brake operation.

Great explanation lstone19. Right now at STM I am mostly qualified on straight air as the older cars use. I hope to one day get qualified on train air also.

 

[lstone19]

Great explanation lstone19. Right now at STM I am mostly qualified on straight air as the older cars use. I hope to one day get qualified on train air also.

Thanks. I started with straight air as well but only single cars have straight air. It has the advantage of giving you direct control of the brakes while automatic air is essentially indirect control - you control the brake pipe and the brake pipe in turn controls the brakes on each car.

 

[joelkfla]

How train brakes work is widely misunderstood. Air does not directly keep the brakes released. Rather, a loss of air in the brake pipe causes pressurized air already in the reservoir on each car to flow from the reservoir to the brake cylinder.

The longer explanation is starting from a train completely empty of air in the brake pipe and the reservoirs on each car, the brake pipe first acts as an air supply charging the reservoir on each car to the brake pipe pressure. Once fully charged (takes a while, particularly for a long freight train), the train is ready to move. At that point, the brake pipe is acting primarily as a control signal. Reduce the brake pipe pressure 10 pounds and the reservoir on each car also reduces 10 pounds by sending that air to the brake cylinder (due to size difference, there's a 2.5 multiplier so a 10 pound brake pipe reduction puts 25 pounds in the cylinder). If the engineer then releases the brakes, the brake pipe starts charging back to the full pressure. As soon as the pressure starts rising, the control valve on each car vents the brake cylinder and the brakes release. Eventually, every thing comes back to full pressure and things are ready to repeat.

It takes time to fully recharge after brakes are released. Repeatedly apply and release and you eventually run out of air. That's why long freight trains rarely make running releases - once you take air, you stop and then wait for the brakes to fully recharge before moving again.

Also, you can't keep taking reductions. Once the air pressure in the reservoir on each car equals the brake cylinder pressure, that's all the braking you can get (except for emergency brakes). If you brake pipe pressure is 70 psi, a 20 pound reduction brings the brake pipe to 50, 20 pounds goes from the reservoir to the cylinder putting the reservoir at 50 psi as well, and the brake cylinder, thanks to the 2.5 multiplier, is also at 50 pounds. That's full service and that's all you can get.

Back to what I said about needing to wait to recharge, if I take a 20 pound reduction, release, and then immediately take another 20 pounds, because things never got significantly above 50, all I can get is 36 pounds in the cylinder (5/7 of 50 - the first application was 5/7 of 70 which is 50). My second application is not as effective as the first.

Leave cars standing without a locomotive and eventually all the air leaks away. That's why handbrakes need to be applied to standing cars.

FWIW, while I am not a licensed locomotive engineer, I am a qualified passenger conductor and interurban operator (same brake system) at the Illinois Railway Museum where I am fully qualified on air brake operation.

Thanks. I followed most of that.

What about modern trams & LRV's, which can run either individually or in trains? Do they use train air, or straight air? And I've sometimes seen PCC streetcars tied together; are they depending just on the brakes in the lead car?

 

[Willbridge]

Thanks. I followed most of that.

What about modern trams & LRV's, which can run either individually or in trains? Do they use train air, or straight air? And I've sometimes seen PCC streetcars tied together; are they depending just on the brakes in the lead car?

I've ridden 1970's vintage trams/LRV's that had straight air brakes, but gradually electric braking superseded air. LRV's typically have three brake systems: dynamic braking, disc brakes, and magnetic track brakes. Regenerative braking can be used where the power network is set up for it.

 

[lstone19]

Thanks. I followed most of that.

What about modern trams & LRV's, which can run either individually or in trains? Do they use train air, or straight air? And I've sometimes seen PCC streetcars tied together; are they depending just on the brakes in the lead car?

I have little knowledge of the braking systems of modern transit equipment. At the museum, our one operable Chicago PCC car is not air nor is any of the recent CTA equipment. One series of older CTA equipment which I do operate has automatic air as I described above just like the interurban cars and standard railroad cars.

 

[BlackDiamond]

PCC cars came in 2 flavors air and all electric. Both types used dynamic braking until the car got down to 10 to 12 mph or so then the tread brakes kicked in, air operated for air cars and electrically applied for all electrics. The latter actually applied the brakes via a spring with an electric solenoid that released the brake. That way the brake was fail safe and applied even if the car lost power. A third braking method was an electromagnetic brake that clamped onto the track mainly used as an emergency brake.

Rapid transit cars of the post ww2 era were often SMEE which was a straight air system with a separate emergency pipe which was fail safe in case of a loss of brake line such as a train parting. They generally also had dynamic braking.

 

[PeeweeTM]

Just to ad some confusion: parking brakes on locomotives and EMUs/DMUs (Electric/Diesel Multiple Unit) in Europe (at least the modern German/Swiss built ones) are of the spring type (Federspeicherbremse).
Air pressure releases those brakes.
They can also be mechanically released, in case of malfunction etc.
Siemens has the best ones; if the engine is dead-in-tow the parking brake releases automagically and it re-applies as soon as the brakepipe AND the brake cilinders are empty. So you don't have to enter the cab to apply or release the parking brakes on those type of locomotives/MUs.

 

[crescent-zephyr]

How train brakes work is widely misunderstood. Air does not directly keep the brakes released. Rather, a loss of air in the brake pipe causes pressurized air already in the reservoir on each car to flow from the reservoir to the brake cylinder.

While this is certainly true for the topic at hand (Amtrak and current mainline railroads) there are random operations like the Walt Disney World Railroad that use straight air.

PCC cars came in 2 flavors air and all electric. Both types used dynamic braking until the car got down to 10 to 12 mph or so then the tread brakes kicked in, air operated for air cars and electrically applied for all electrics. The latter actually applied the brakes via a spring with an electric solenoid that released the brake. That way the brake was fail safe and applied even if the car lost power. A third braking method was an electromagnetic brake that clamped onto the track mainly used as an emergency brake.

I didn't realize PCC cars used dynamic braking! That's really cool. I'll try to pay attention to that next time I travel on one.

 

[Willbridge]

...
I didn't realize PCC cars used dynamic braking! That's really cool. I'll try to pay attention to that next time I travel on one.

Also trolley coaches.

 

[BlackDiamond]

I didn't realize PCC cars used dynamic braking! That's really cool. I'll try to pay attention to that next time I travel on one

My one time operating a PCC under shall we say very informal circumstances on route 50 in Philadelphia I recall there was a bit of a lag from when you pressed the foot pedal to when the brakes came on. I guess an operator would have to anticipate his/her stops. Fortunaletly this was late at night and little traffic.

On railroad locos I believe that lag is much longer in the order of several seconds before dynamics kick in.

 

[PeeweeTM]

On Siemens Vectron and Bombardier Traxx dynamic brakes react as fast as air brakes. They just lack force when below a couple of mph.
On Dutch diesel DE6400 the dynamics build up slow, so braking starts with air and after a couple of seconds the dynamycs (ED-brake) blends in. Below about 20 mph dynamics blend out again, air takes over.

But overall you're correct; brakes on trains react slower then what I notice on street cars.

If you have some dollars to spend, the book "Management of train operation and train handling" might be interesting.

 

[caravanman]

When I worked at British Rail, in the 1970's, most of the locomotive hauled passenger trains used a vacuum brake system, rather than an air brake one. Were early US passenger trains also vacuum braked, or has air always been used?

 

[jis]

When I worked at British Rail, in the 1970's, most of the locomotive hauled passenger trains used a vacuum brake system, rather than an air brake one. Were early US passenger trains also vacuum braked, or has air always been used?

India used to be vacuum brake too. But they have been converting to air brakes (and also to a version of tightlock AAR CBCs from chain link couplers) on their Broad Gauge rolling stock. Most locomotives at one time were capable of handling both braking systems and both couplers. I don't know if they have finally dropped the older stuff on newer engines.