looking for value that determines how quickly a train can be slowed/stopped.
assume the brake sizes depend on the weight of the car. Wondering if there is an FCC guideline for this?
greg - Philadelphia & Reading / Reading
I imagine it is more up to physics than a regulatory agency. But isn't the FCC the Federal Communications Commission?
Rio Grande. The Action Road - Focus 1977-1983
Probably should be researching the FRA (Federal Railroad Adminsitration) site
i was thinking federal Commerse commision. I see there is a Federal Railroad Administration.
the regulations would dictate the design of the brake to meet stopping distances
i found the following:
According to the National Safety Council:
since there are brakes on every car, i would expect stopping distance to be independent of train length. Is the reason an average freight train takes 18 football fields to stop because of delays in brake pressure propagating down the lenght of a train?
Not dependent on length, but on mass, momentum, condition of rails, condition of brakes, initial speed at application, how strong an application............................
The reason a train takes so long to stop is due to traction, friction by the brakes, and not wanting to stop the wheels so that the tires slide and flatten eventually. The latter would greatly add to the risk of derailments and potentially more catastrophic results. Trains should not skid to a stop.
Physics.
A 1.75 ton automobile travelling 55 miles per hour is 479,868 joules of kinetic energy.
An 18,000 ton train travelling 55 miles per hour is 4,935,786,815 joules of kinetic energy.
All braking energy must be dissipated as heat from brake shoes, wheels, etc. (Ignoring dynamic braking.)
Auto braking is also more efficient because of the friction of rubber tires on the road versus steel wheels on rail. If train wheels "lock up", they slide on the rail and the train may actually take longer to stop, as I understand it (not to mention ruining the wheels).
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Look at it from the other direction - the reasons a train is more efficient than trucks or buses is because of the relatively low friction compared to rubber tires on asphalt or concrete roads. Takes much less power (and thus much less fueal) to pull a heavy train than anything approaching the equivalent load on robber tires on a road. But then when you try to stop - you have the opposite problem There is not as much friction to stop that weight, and the momentum is incredibly much greater. Just one loaded freight car - 100 ton car, you're looking at 50-75 times the weigh of one passenger car. Nearly 100 times if talking about a smaller lightweight car, though most are over 3000 lbs these days (and some wonder how we had simple cars 40 years ago that got 50MPG, no hybrid or special stuff...).
The brakes on modern trains apply pretty darn quickly. Those rear-end devices are more than blinky lights, they monitor brake pressure and transmit that to the loco cab and in many cases, in an emergency application open up the rear end of the brake pipe too, speeding up the application.
Pretty much all comes down to simple physics - what's the kinetic energy of a 100 car loaded freight train plus the 3 locos on the front pulling it, moving at 55 mph? 100 tons per car plus the empty weight of the car. Full brake application at speed will have some if not all the wheels sliding, there's no ABS on freight cars.
Since Byron snuck in and sad the same thing - yes, sliding is worse. That's why ABS was pushed in cars, on MOST surfaces, maximum braking force comes with the wheel and road surface right at the threshold of loss off traction. Slide the wheels and the braking force is reduced, makign it take longer to stop. Not to mention absolutely no directional control if it's the steering wheels that are locked up. Non-issue with trains, but the sliding versus just about sliding still applies.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
gregcsince there are brakes on every car, i would expect stopping distance to be independent of train length.
i believe i understand the physics. i understand that there's one set of locomotives acceleration a 5000 ton (100 x 50 ton cars) train.
but there are brakes on every car ... twice the number of cars, twice the number of brakes
gregc A light rail train requires about 600 feet to stop - the length of two football fields. Compared to this, the average freight train we mentioned above traveling at 55 miles an hour may take the length of about 18 football fields to stop.
i'm not asking about the difference between cars, trucks and trains, i'm asking about two trains of different lengths .. and i'm asking about stopping, not starting.
my question is why does a light train take less distance to stop than a average freight train, assuming by light they mean fewer cars (and fewer brakes).
gregci was thinking federal Commerse commision.
You were thinking the INTERSTATE Commerce Commision, predecessor to the FRA.
I know of no regulations that specify the "braking force" required on a car or engine. There may be requirements for air pressure, brake cylinder size, piston travel, type of brake shoe, etc. but not braking force per se.
There are also no regulations regarding "stopping distances", except that a train must "stop before passing".
Dave H. Painted side goes up. My website : wnbranch.com
gregcgregc A light rail train requires about 600 feet to stop - the length of two football fields. Compared to this, the average freight train we mentioned above traveling at 55 miles an hour may take the length of about 18 football fields to stop. i'm not asking about the difference between cars, trucks and trains, i'm asking about two trains of different lengths .. and i'm asking about stopping, not starting.
A "light rail train" from the page you are quoting is something like this:
... not a "light" freight train.
Much less mass, therefore much less kinetic energy, therefore much less time needed to stop than a typical freight train.
I believe that the light rail vehicle also has the benefit of regenerative braking on all the powered axles.
gregcmy question is why does a light train take less distance to stop than a average freight train, assuming by light they mean fewer cars (and fewer brakes).
Part of it is a smaller train has the brakes apply faster because it takes longer for the brake application to propigate through the train line. The longer a train is the lower the pressure is on the rear cars, so there is less air pressure in the brake cylinders, there is a gradient to the braking force across the length of the train. The longer the train the more the gradient.
We did a test of an 18,000 ft train that had 4 sets of engines, a set head and rear and a set every 6000 ft. The report on the performance of the train was that it accelerated and stopped like a 6000 ft train. My takeaway was that the reduced distance the brake application had to propigate (3000 ft) and the reduced pressure gradient (air supply every 3000 ft) made it react like a smaller train.
dehusmanPart of it is a smaller train has the brakes apply faster because it takes longer for the brake application to propigate through the train line.
thanks dave. that makes sense
cuyamaMuch less mass, therefore much less kinetic energy, therefore much less time needed to stop than a typical freight train.
but if each wheel has a brake, for each additional car and increase in mass there is proportionally more braking force.
Here's a study that might be helpful:
https://ntsb.gov/news/events/Documents/2017_casselton_BMG_6_Casselton-Brake_Study.pdf
Notice that one way they are reducing stopping distance is to decrease the propagation time, through DPU or ECP (electronically controlled pnuematic) brakes.
Dave,
i finally figured out that Car NBR stands for Car nominal braking ratio, which is
car net brake shoe force car gross rail load
but pg 76 reports that the max acceptable loaded car NBR is 14%. Doesn't this limit the braking and therefore maximum deceleration.
why? is this to deal with the propagation delay and the cars later in the train will not be braking as quickly?
That's part of it, but also the available brake force does not go up in direct proportion to the mass increase - perhaps a more favorable ration by tackign on a lot of empties, but for loaded cars - you're going to end up with less braking force per unit of mass of the train as it gets longer.
As to the 'why' there is a limit - again it comes down to the coefficient of friction between the brake shoe and steel wheel, and between the steel wheel and the steel rail.
ANd yes, the brake propogation delay comes into play - notice what often happens if the head end of a train derails and stops far quicker than any brake application? the rest of the train just keeps on coming until all the momentum is absorbed and whatever remains finally stops. If the brakes apply too much stopping force on the head end before the tail even sees the pipe reduction to set ANY brakes, you have a similar effect.
rrinkerANd yes, the brake propogation delay comes into play - notice what often happens if the head end of a train derails and stops far quicker than any brake application? the rest of the train just keeps on coming until all the momentum is absorbed and whatever remains finally stops.
Just this morning in Wellington, Ohio on the CSX:
Some outtakes from a 1955 Air Brake Association Convention:
Brakes_intro by Edmund, on Flickr
Brakes_intro_0001 by Edmund, on Flickr
Brakes_P-S by Edmund, on Flickr
Brakes_P-S_0003 by Edmund, on Flickr
Brakes_P-S_0004 by Edmund, on Flickr
If this is helpful,
More can be found here:
https://www.flickr.com/photos/gmpullman/albums/72157708812136901
Thank you, Ed
gregc: "my question is why does a light train take less distance to stop than a average freight train, assuming by light they mean fewer cars (and fewer brakes)."
Weight ! (and... better brakes on the passenger cars, in most cases)
rrinker That's part of it, but also the available brake force does not go up in direct proportion to the mass increase - perhaps a more favorable ration by tackign on a lot of empties, but for loaded cars - you're going to end up with less braking force per unit of mass of the train as it gets longer.
the maximum tractive effort of a locomotive is typically 1/4 the weight on drivers. I see little point in designing a locomotive that provides more tractive effort than this amount (although they do).
similarly, there is little sense in designing brakes on a freight car that can provide a combined braking force on all its wheels that exceeds 1/4 the loaded weight of the car.
unfortunately, as you say, that amount of braking on an unloaded car would cause the wheels to slip. So do you design for the loaded or unloaded weight?
rrinker As to the 'why' there is a limit - again it comes down to the coefficient of friction between the brake shoe and steel wheel, and between the steel wheel and the steel rail.
there's the distinction between physics, good practice and regulations.
If 25% the loaded weight is the max useful braking force, why does the report posted by Dave suggest 14%? (best practice in an emergency)?
ironically, page 7 posted by Ed suggests between 50 and 75%.
thanks again Ed for posting such information
OldEnginemanWeight ! (and... better brakes on the passenger cars, in most cases)
if each freight car has 8 wheels with brakes, a 100 car train has 800 brakes.
if each car can provide a braking force of just 10% its weight, a 5000 ton (100 x 50 ton) train could have 500 tons (1,000,000 lb-force) of braking force.
gregcif each freight car has 8 wheels with brakes, a 100 car train has 800 brakes.
One also has to consider the era. A 1950's train might have 100 tons of capacity distributed over two 50 ton cars, giving you 16 brakes for 100 tons, while a modern train has one 100 ton car, giving you 8 brakes for 100 tons.
my take was that the Nominal Brake Ratio was per car. If the fully loaded car is 40 tons, then the brake for each of the 8 wheels is 0.5 tons if NBR is 10%. If the car weighs 80 tons, then the brake force per wheel is 1 ton.
And braking systems have improved since the 60's as well. As modelers we tend to think of the distinction being between K type and AB brakes - but ther have been MANY finer upgrades to the basic AB system ooover the years, mostly aimed at improving the propogation delay for booth set and release as well as in emergency applications. Since most of these changes are in places yoou can;t see - internal int he brake stand in the locoo, internal in the control valves on each car, etc - it's noot something most modelers cooncern themselves with, because, well, there's nothing really different to model, apart from some fine print decals on the car.
Commuter and light rail often has things like electro-pneumatic brakes, since even for a long train, the electrical signal will effectively apply the brakes instantly on every car. I don;t see something like that coming to freight cars any time soon, you'd need too hook up an electric line in addition to the air hose when coupling cars, and discoonnect it when setting out - air hoses can handle just being pulled apart, would need a rahter interesting connector and cable to allow an electric signal line to do the same. And one side has to be a plug and one side has to be a socket - contacts on an electrical cable on a car sitting at an industry being loaded or unloaded, how do you stop corrosion of the contacts?
rrinkerAnd braking systems have improved since the 60's as well.
A big improvement was the switch to composite brake shoes. Less heat, less fade, greater milage, fewer adjustments to cylinder travel.
rrinkerCommuter and light rail often has things like electro-pneumatic brakes, since even for a long train, the electrical signal will effectively apply the brakes instantly on every car.
I saw a demonstration of the electro-pneumatic system at a BNSF training center in Overland Park, Kansas. It had real promise, mainly for unit trains, but, as you say, limitations, too. The big advantage was you could keep the brakes applied while recharging the reservoirs.
E-P brakes have been around since the mid-1930s. Many of the early streamliners were supposed to be so-equipped. Some got it, most didn't.
Cheers, Ed
now i'm curious. How do electro-pneumatic (E-P) brakes work?
is there an electric circuit down the train from car to car, presumably at some max voltage to disable the brakes and enabling brakes to various degrees with lesser voltage? presumably if theres a break in the connection, all trailing brakes get enabled?
and the advantage is that brakes are applied to all cars at the same time?
gregcnow i'm curious. How do electro-pneumatic (E-P) brakes work?
https://www.hsdl.org/?view&did=15763
Good Luck, Ed
Tantalizing with a lack of details - but that PDF talks about two way communications over the ECP circuit and all sorts of controol commands that can be sent to each car - my guess it would use a singalling system such as RS485 to get the robustness of signal, the multiple drops, and the ability to run long enough signal cables to handle long trains.
Or perhaps a more robust but similar system with higher voltage as well as higher current to be able to support enough 'stations' (cars), since standard RS485 would never handle 200+ cars in a train.
It's really not an issue with light rail cars operated in MU, they aren;t typically switched ina nd out in rando places, rather the sets are configured at the shops and then sent out for service, but that sort oof thing wouldn;t work for a general freight train, there needs to be a robust coupling system between cars for this to work on your typical freight train.
gmpullmanI saw a demonstration of the electro-pneumatic system at a BNSF training center in Overland Park, Kansas. E-P brakes have been around since the mid-1930s.
E-P brakes have been around since the mid-1930s.
gmpullmangregc now i'm curious. How do electro-pneumatic (E-P) brakes work? https://www.hsdl.org/?view&did=15763
i don't think this is a description of E-P brakes you saw demonstrated nor what could have been around since the 1930s. this is a description of a study of Electronically Controlled Pneumatic (ECP) brakes from 2006.
I don't see much discussion of technology besides the implied benefits of smaller electronics replacing "pneumatic logic". I did read the following benefits
but it seems there are significant economic hurdles that challenge such an approach
gregci don't think this is a description of E-P brakes you saw demonstrated nor what could have been around since the 1930s.
OK, I guess I'm mistaken. Sorry.
I was, at least, insightful enough to have made a short video of the demonstration. I'm glad I wasn't imagining the whole thing! I just posted the video to YouTube.
There is a short chapter regarding Electro-Pneumatic Brakes (ECP) in the excellent book Train Wreck by George Bibel published by Johns Hopkins University. ISBN 978-1-4214-0590-2
Mr. Bibel also mentions the "Burlington Brake Trials of 1887. Several manufacturers submitted various E-P designs, including Westinghouse. It was basically the same as the system proposed today. The dilemma then was how fragile the electrical apparatus was in the environment of the freight trains. This has been overcome by solid state electronics. However the cost of equipping some million-plus freight cars has not yet been addressed.
I suggest you purchase a copy of the book, it will answer many of your curiosities.
In 1937 most, but not all, of the cars built by Pullman Standard for the Twentieth Century Limited were equipped with a form of Electro-Pneumatic brakes. New York Central dropped the plan and never equipped the locomotives with E-P equipment.
The Florida East Coast and perhaps Atlantic Coast Line had a variation of E-P brakes as well.
THIS site explains some differences between E-P brakes and ECP brakes:
http://www.railway-technical.com/trains/rolling-stock-index-l/train-equipment/brakes/electro-pneumatic-brakes-d.html
Ed,
thanks for the info. I appreciate your willingness to answer my questions.
the Railway Technical site you linked to makes the distinction between Electro-Pneumatic (E-P) and Electronically Controlled Pneumatic (ECP) brakes.
it says E-P brakes were tried on NY subways in 1906. It looks like there were 2 electric circuits down the train. One opened a valve to increase brake pressure and a 2nd opened an exhaust valve to decrease brake pressure. Air pressure remains constant.
that link's description of ECP says it uses microprocessors. It also says that only 25% of the communications bandwidth is used for braking and ~60% for non-braking related sensors such as bearing (temperature?), reefer temperature and tanker pressure.