Braking Long Trains

I've been meaning to ask this question for a while now. Why do long trains take more time to stop? I know at first it seems like a stupid answer. Obviously, its because they weigh so much more. But each car has it's own brakes, don't they? To the braking power to weight ratio of a single car, with one set of brakes, should be the same as 100 cars with 100 sets of brakes, right? So a long train should be able to basically stop on a dime. Obviously they can't so my theory has a nice big whole in it somewhere
Whats the deal?

Longer trains take longer to stop because it takes longer to set the air.
You can't just take a max reduction on brakes without having the train either derail or break in two.
All the air venting from brake pipe has to vent thru the main brake valve on the locomotive, by doing it to fast the front of train will have brakes and rear will still be in release or at minimum brake.

The responder's anwer is essentially correct, except for:

DutchRailnut wrote:All the air venting from brake pipe has to vent thru the main brake valve on the locomotive...

This is not precisely correct. Modern car mounted control valves have a feature called "accelerated service application," whereby a portion of the brake pipe reduction is triggered and vented locally, on each car. Cars with longer sections of brake pipe are also equipped with service vent valves that do the same thing -- all in the interest of accelerating the application of brakes throughout the train.

The main factor in considering train braking is the tons per operative brake ratio. It's not the number of cars that matters the most -- it's the mass involved. The more tons per brake, the longer the stopping distance. So -- if you have a longer train with mostly empty cars, it will stop in less distance than a shorter, heavier, loaded train.

By far, though, the largest factor in train braking is the speed of the train. If you have two identical trains on identical railroad grade, with one going 30 mph and the other going 70 mph, the 70 mph train will require almost 3 times the distance to stop.

LCJ thanks for additional info, do most freight cars these day have empty/load sensing valves to determine the brake rate for each car ??

Wabtec's website shows the empty/load valve as part of the standard system for a freight car. It's number 6 on the diagram here:

http://www.wabtec.com/railroad/coal.asp

On the CN we've got several units that are equipped with 'Trainlink ES' head end boxes, combined with an ES marker you get additional brake pipe venting from the marker if desired. Of course even if you have both the head end box and marker it still doesn't work half the time, but when it does it helps stop a little quicker.

Yessir! Here it is:
http://www.wabco-rail.com/main/product. ... duct_id=52

That's pretty nifty.

AKA "quick-service", and the opposite side being, of course, the "accelerated-release" feature, if I recall the teachings of the sage LCJ.......

No matter how much I try to forget it all, it keeps coming back to me.

It seems Westinghouse has perpetually been trying to accelerate one thing or another.

LCJ wrote:No matter how much I try to forget it all, it keeps coming back to me.

It seems Westinghouse has perpetually been trying to accelerate one thing or another.

Now if they could just develop a telemetry system that never looses communication!

GA or LCJ, either of you guys ever have the chance to operate an ECP train?

I have not. But then, they don't let me run trains anymore.

1008, the above info from LCJ and others is one reason the railroads are exploring ECP brakes. If I understand their operation correctly, the brakes themselves still apply and release with air like regular brakes, but the control of the valves is done electronically. The "instructions" are sent to each car's valve through wire, so the brakes on the 92nd car begin applying at the same time as the 14th car or whatever.

Did you ever stand next to a long train when they made a cut near the head end? You can hear the "sneeze" of air coming, going by you, and traveling toward the back. On a long train it can take several seconds to reach the rear. And if you are venting air at a service rate rather than emergency, and allowing time for each car's piston and levers to move, taking out the play, moving the shoes up to the wheels, you will see that it can be quite a while after the engineer makes a brake application before that train even STARTS to slow down.

I believe ignorance of this by the public is a factor in some grade crossing accidents. People think that if they cut it too close, the engineer can somehow "jam on the brakes" and give them an extra half-second to make it across. In reality even if the hogger dumps it several seconds before the crossing, train speed will not even begin to decrease before the power enters the road.

Hi All,

A common "trick" is to use additional engines with dynamic brakes on the tail end of the train. This is fairly common when long trains are descending Cajon Pass. As the train goes by, you hear the growling of the dynamics on the trailing engines. You also hear the squeals from the brakes on the individual cars. Even then, some trains have been known to get away from their engineers on the way down with disasterous results.

Fred Carlson & Brian Smith, in Railway Age wrote: ECP braking gets results Railway Age, Oct, 2000

Field trials in Canada and Australia are illustrating the benefits of electronically-controlled pneumatic braking systems.

The Quebec Cartier Mining (QCM) Railroad and the BHP Iron Ore Railroad in Australia have been operating electronically-controlled pneumatic (ECP) braked trains in revenue service for several years. Data collected from these operations is being used by Transportation Technology Center, Inc. (TTCI), Pueblo, Colo., to evaluate the economic benefits of ECP brake systems. Analyses of both systems indicate ECP brakes offer many economic and safety benefits over conventional brake systems.

The QCM experience has confirmed many of these benefits;

* Continuous reservoir charging, which makes it harder to "run out of air" on descending grades.

* Graduated release, which eliminates stalling on descending grades, enables running brake releases from any speed, and sharply reduces the need to power brake.

* Reduced fuel consumption due to improved train handling and, to a lesser extent, reduced air demand.

* Stop distances 30% to 70% shorter due to simultaneous application and release of brakes.

* Reduced slack action due to simultaneous application and release of brakes.

* Reduced wheel, brake shoe, and draft component wear due to reduced slack action.

The BHP Iron Ore system has been operational for a shorter time and the benefits are not well quantified yet. However, the expected improvements in stopping distance have been measured.

The greatest difficulty (as I see it) will come with trying to implement this system in non-unit train service, where cars are routinely separated and switched along their route.

LCJ, if I read your post right, there is more to the ECP brake than just electronic control of the valves. Graduated release and continuous charging - sounds like a good thing. I stand corrected on my previous post saying it was essentially the same as existing technology.