Is a freight trains length limited by how much force a coupler can take?

Question

I see these massively long trains and wonder if the length of a freight train is limited by the amount of force that a coupler can take? Is this a factor that limits the length of a train, and how often do these things fail? I've seen some really decrepit looking freight cars near the front of a long train and have wondered if they might be the "weak mechanical link". Thanks.

Answer 1

No, weight is limited by coupler strength, length is mostly limited by physical limitations of the territory and air pressure at the rear of the train.

Imagine a 130 car train of all empty cars, it wont weigh very much but will be over 7000 feet long.

Now take that same train filled with ore or coal and it will weigh too much to pull without breaking the couplers.

Another factor is the curvature of the track, stronger couplers would not help there, it happens more than you might think that train cars are "stringlined" where the locomotive is pulling so hard that light cars hop off the rail and are pulled to the inside of a track.

Simple physics require that trains being pulled want to be straight and will do everything to become straight.

Andy is right about DPU (remote controlled locomotives placed in or at the rear of a train) changing the picture. They can increase air brake efficiancy and reduce coupler forces.

Answer 2

Train Couplers

Answer 3

I agree with Andy and Rango, also it used to be common for a knuckle to break, because I used to have to change 5 or 6 a year when I was a Brakeman. The reason being that the Railroad made us pull trains that were under powered and over weight. Another way to break a knuckle was coupling up in the dark and not paying attention to make sure the coupler was open.I've did that a few times. Every locomotive that I was on had a spare knuckle and spare air hose with coupllings.

Answer 4

They aren't limited by length.They are limited by the amount of weight they are pulling.Now that we use so many DPU pusher locomotive configured trains the weight they can handle has gone up.No matter how decrepit a car looks it still has at least standard strength couplers on it.A standard coupler has a knuckle that can withstand 230,000 pounds of force on it before it fails.

Edit PM Railfan the knuckle is supposed to be the weakest link in the coupler so it fails before the coupler itself.They do have high strength couplers that are used mainly on ore,coal,and grain cars.They are fitted with E50HT knuckles that allow higher draft forces.The exact number on the escapes me at the moment but i believe it's around 400,000 pounds.I remember in simulator training many years ago one of the things they taught up was starting a heavy train(like a 14,000 ton grain train) on a 1 to 2 % grade.The computer had a graph line to show draft forces.It also had a baseline set at 230,000 pounds representing the coupler limit.The draft forces would go over that line getting the train moving.It would go as high as 270,000.You had to be very patient to get all the speed out of every throttle notch before advancing the throttle.The forces would be so high that if you tried to advance the throttle past about run 4 it would break the train in two.

Answer 5

depends on how fast the the train takes up the slack in the train most railroads the take up slack is 1 mph more than that will cause a problem any more that with the forces multiplying between the cars and break the couplers

Answer 6

Yes. The couplers can only handle upto about 14,000 tons. Once the train starts to pull, move the train. If the engineer is not careful with his train handling it is very likely to break the knuckle. The length of a train really has nothing to do with the weight, it is mearly the weight of the train.

Answer 7

rango and andy have this one. its not length, but weight.... the amount of forced 'pulled' on two adjoining couplers. ofcourse lengthening a train adds weight, but you can get a longer train with empty cars than you can loaded. still, it comes down to how much force are you excerting on the coupler, thats done by the weight, not length. personally i would have thought it was higher than the 230g pounds stated above.



i wonder what 500g pound couplers would do to the configuration of trains???

Answer 8

length is limited by how many locomotives are on the train, couplers do fail but not very often