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

Yes, there are limitations.

“Standard” draft gear, including the “knuckle” is rated for a safe maximum of 240,000 psi. Some hopper cars designed for grain or coal loading have a more heavy duty draft gear arrangement that allows for pulling greater tonnage. It is tonnage that is the determining factor. Consequently, the longer the train, the more tonnage and resistence through curvature there is. Length alone, however, is more a factor when considering the air-brakes.

Various portions of the draft gear can fail. The knuckle can break and the drawbar can break. Some times these failures are due to an old, partial break then give up the ghost at a later time. In most instances, however, the failure of the draft gear is the result of improper train handling or an emergency application of the brakes. In emergency, the engineer’s ability to control “slack action” is minimal, especially when running with the slack in “buff,” that is to say, bunched up, such as when running in dynamic brake. In days of long ago, “break in two” reports had to be filed when they occurred, just as today. But, in those days, the engineer could be disciplined for a break in two that he had caused. So, in a little known trick, the old hoggers would carry pineapple juice in their grips. When poured onto a fresh break, it would darken the exposed shiny metal of the center of the failed piece of equipment, making it appear to be an old break. I’ve never tried it to see if it works, but that’s the story.

A train, as with any chain, is only as strong as the weakest link. Of course the highest levels of force are encountered with the first car coupled to the engine. In your observation of the dilapidated looking equipment you have seen, cars must be retired by mandate at 50 years of age. In addition, every ten years they must undergo “COTS,” the anagram for cleaned, oiled, tested and stenciled. Freight cars get beat up during their service life, especially gondolas. They may look un-rail-worthy, but they are in compliance with Federally mandated conditions.

Helper engines, either manned or running remotely controlled, are placed in the train or at the trains rear when tonnage or grade will cause draft or buff forces to be excessive of the draft gear’s capability, reducing the amount of load nearer the head end of the train, where forces are highest.

In a worst case scenario that I have ever seen was a car whose draft gear had been gutted due to an emergency application and ripped the end of the car off with it.. It should have caused a derailment, but everyone got lucky. Derailments do occur as a result of failed draft gear, however, which is why the carriers take a keen interest in when they happen, or in an engineer with whom at the controls they happen frequently.

Addendum: I smoke Camels. On which planet are they running these million ton trains? If it's not too far, I'd pay to see one under way. If you are visiting the planet soon, maybe I can hitch a ride with ya...........

Answer 2

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 3

at speeds as low 5 mph knuckles have cracked in the pull face. but the coupler is very unlikely to fail on its on unless the yoke is cracked, or the draft gear is broken or fouler blocks are missing
or the siding center sill/ E.O.C is jammed . now rotary coupler often crack at the head, but all freight are inspected by carman
most derailments are cause by truck failure thin flanges, slid flat wheels , bolter bowl is broken , the knuckle will fail first they why
tank cars must have double shelf couplers. your answer is no 108 cars is 1 mile long

Answer 4

I don't know what these other guys have been smoking but one empty box car can weigh up to 70 to 100 thousand pounds! One train can weight up into the millions of tons.