How is a train able to pull so much weight?

Question

I have lived by trains nearly all my life and still can not believe the amount of weight they can pull....the largest truck can only pull so much weight but a train can pull dozens and dozens of cars filled to the brim with stone, weighing over 50 tons each! With all that pressure pushing down on each car, how does the engine (weighing a small fraction of the total weight, pull all that weight? Especially getting started slowly with virtually no grip - just smooth metal on metal. Always wondered!

Answer 1

Good question, with some good answers above.

For one thing, as you pointed out in your question, smooth rail under steel wheels. Though this would seem unlikely for traction it is exactly what make rail cars so efficient. There is minimal contact area with the wheel/rail, so there is minimal friction, unlike a highway tire on a roadway.

In addition, the cars are mounted on roller bearings in the axle journals, again providing minimal resistance.

Since gravity and horsepower are constants, there is a simple formula used to calculate how much horsepower will be needed to move a given amount of tonnage up a grade, called the "Rolling Train Resistance Formula". It is as follows: take the horsepower per ton (HPT), multiply by twelve, divide by the percentage of the grade, and that will tell you the speed you will make. Expressed another way, HPT x 12 / %G = S.

The locomotives are indeed putting out a lot of horsepower. On US railroads a typical diesel-electric road locomotive develops between 3,000 and 6,000 horsepower each. But this applies to the diesel engine itself, or prime mover, which powers the main alternator, companion alternator and auxilliary generator.

The main alternator supplies electricity for the axle hung traction motors, which are what does the actual pulling. "Tractive Effort", or torque, is the name of the game, and the electric motor is king in this realm. A simple rule of thumb is that the locomotive will convert approximately 25% of its weight into tractive effort. Again, most locos in the US are in the neighborhood of 410,000 lbs., they produce about 105,000 ft/lbs of torque. To compare, the production model of the 429 Hemi, gasoline powered automobile engine, was the highest producer of torque at 405 ft/lbs.

As far as getting the power to the rail, the weight of the locomotive is where it gets its traction. When 200+ tons occupies twelve points on the rail, each not much bigger than a silver dollar, it "hooks up" pretty good. In instances where extra traction is needed, such as at starting or on wet rail, sand is applied in front of the drive wheels for extra grip.

In addition, there is "slack" in the draft gear between each car, so as each car starts, it starts out the one behind it, and so on. If the tonnage was a solid "chunk", no one is going anywhere. The law of inertia dictates a body at rest tends to stay at rest, while a body in motion tends to stay in motion.

So there you go. The real trick is getting the thing STOPPED where you want it to, but thats another question.

Addendum: I have been a denizen of the rail category for a few weeks and I must say I am amazed at the interest and response this question has gotten. 80 answers is incredible, and I think xyz hit it right on the head with his commentary.

I hope these people return to this category as it is usually interesting, informative, sometimes humorous or entertaining and at times, deals with some very important issues.

One other comment, from Nyl, is a very nice sentiment. Railroaders are some of the hardesting working people around. Most see us at grade crossings, sometimes waving or smiling, and assume the operating personnel are along for the ride. Most don't see them at 3:30 in the morning, wading through two feet of snow to tend to their duties. There are other rigors of the service as well.

But it is those who support us as operating crafts, that are the most overlooked. Electricians, machinists, laborers, pipefitters, boilermakers and others, none the least of which is the maintainance of way personnel. You should see the latter spend sometimes two or more days of non-stop work at a derailment site to get traffic moving again. And that is even if it is in two feet of mud, 115 degrees of summer heat or temperatures well below zero. These are true unsung heroes. And without them, those engineers aren't going anywhere....................

And, it's not just railroaders that are largely unsung. Everyone in the transportation industry, truckers, pilots, UPS and FedEx, Merchan Marine right down to the city cabbie provides the invaluable service of literally keeping this nation moving, often times in unpleasant or dangerous situations.

We have labor day, as general recognition of all who participate in the workforce, as well as Boss' day, Secretary Day, etc. How about a new National Transportation Worker Day?

All of August is open and another great opportunity for Bar-B-Q, beer and high explosives. Another 4th of July.................

Answer 2

Mostly it's low rolling resistance. However the rolling resistance isn't really provided by friction. It's instead provided by deformation in this case deformation of the wheel and rail. For example a car with a flat tire is a lot harder to push than one with all four tires inflated even though the flat tire actually has less friction with the road. This is, I believe, the main reason why the mainline railroad tracks that get the most use and will be used by the heaviest, fully loaded trains are made from heavier, sturdier rail than are the sidings and yard tracks. It's so that the rails will flex or deform less as the train rolls over them deceasing the rolling resistance and making it easier for the locomotive(s) to pull the train. If a 100 car train had absolutely no rolling resistance and was on perfectly level rail then you could get the whole train moving by your self without even pulling very hard. However it would take you a very long time to get the train moving even a couple of miles per hour.

Wow! Some really crazy answers come after mine. I just want to say a few more things here. The engines don't have gears like a car. Just one gear ratio in the final drive which is fixed. You can do this because the electric motors used to move the locomotive have a really flat torque curve compared to the diesel engine that ultimately powers them, and they'll still produce torgue even when completely stopped. This eliminates the need for a limited-slip device i.e. a clutch which would dissipate a lot of energy and wear out really fast due to the fact that it takes so long to get the train moving a decent speed. There's other advantages too such as being able to simply and accurately guage your power output without any loss of power.

Answer 3

The cars themselves are not hard move. Once, long before I became a broken down old fart, I worked in a Juice plant. A train car had come in that needed unloading, but it was not properly aliened with the warehouse door, so we released the brakes and nine of us pushed it into position. On level surfaces, they are easy to move due to having steel wheels on steel rails. As for going up hill, you have likely seen 1 or 2 locomotives at the end of a long train. The is a pusher unit, meant only to push long trains over hills, at which point they detach and return to their idle sidings. As a cab driver, I've frequently carried engineers out to sidings to change out crews.

Simply put, each locomotive is a big power plant, with electric motors on the wheels. Some times you will see a train pass, you will note a power unit not putting out smoke, or it is just half the unit. This is because they have plenty of power from the other units, but they need extra weight on powered wheels to pull the train.

This would be a good time for everyone to learn more about how trains work, as the rail companies wants to make changes in the make up of engineers. Instead of three, they say there only needs to be one engineer per train. All the millions they make and they want even more by increasing the risk.

Answer 4

1. Train tracks are very close to level while roads have steeper hills to climb and go down.

2. Locomotives have a lot of gears to go through. You may see a heavy truck on the road that when he pulls from a red light he will go through two or three gears and still be in the intersection. Those low gears give him the torque he needs to get rolling. Same with a train. Trains take a long time to accelerate because of that heavy load too.

3. Brakes!!! It takes miles to stop a speeding train. They have excellent brakes on each car, yet the load is so heavy that it still takes a long time to stop. That is why ALL trains have the right of way.

4. Though the engine is lighter than the rest of the cars put together, it is the one with the driving wheels while all the other car wheels are free wheeling. The engine, though, is the heaviest or one of the heaviest parts of the train and all that weight is baring down on the metal wheel. With only a few square inches of contact, through the wheel where they touch the track, all the weight of the engine is rest on the rails. It would take more power than that of a locomotive to overcome the friction that is developed between wheel and rail at that much pressure.

Answer 5

Don't forget about the weight of the Engine. Sometimes they have to add Engines to the same train, in order to pull the weight. Sometimes as much as 80 or 90 cars. The wheels of all those cars are very well oiled, so they do move freely.
There is no lifting involved here and it is much easier to move a heavy weight on wheels horizontally that vertically. What they have to worry about is hills and it is all figured mathematically, Horsepower and weight. You would be amazed at what a 100 pound dog can pull in a wagon.
The Engines are just that. All Engine and the horsepower developed by those diesels is phenomenal.
You posed very interesting question, it is nice to see one for a change.

Answer 6

Wow! Some really crazy answers come after mine. I just want to say a few more things here. The engines don't have gears like a car. Just one gear ratio in the final drive which is fixed. You can do this because the electric motors used to move the locomotive have a really flat torque curve compared to the diesel engine that ultimately powers them, and they'll still produce torgue even when completely stopped. This eliminates the need for a limited-slip device i.e. a clutch which would dissipate a lot of energy and wear out really fast due to the fact that it takes so long to get the train moving a decent speed. There's other advantages too such as being able to simply and accurately guage your power output without any loss of power.

Answer 7

Truck Pulls Train

Answer 8

The key to this is the low rolling resistance of a steel wheel on a steel rail. The rolling resistance coefficient is approximately 0.002.
That means that a train will roll down a gradient of only 1 in 500.
It also means that to pull a 50 ton car requires only 224 pounds of pull, which is why a strong athlete can pull one.
So a 20 car train needs a pull of 4480 pounds or two tons to roll it at a steady speed.
The only extra pull you need is to accelerate the train, to overcome air resistance, and to climb a gradient.
The other answers talk plenty about the power available from railway engines.

As for the grip required to produce the traction, the slip friction coefficient of steel on steel is about 0.3.
So the basic two ton pull can be produced by an axle with only 7 tons load on it. Locomotives are very much heavier than that, in order to provide traction for acceleration and hill climbing.

Answer 9

The wagons have solid steel wheels and are on a steel track, so the friction is very low, and presumably the bearings are also low friction as well. My experience with driving buses has lead me to believe friction is non - linear, it goes down as the speed goes up. So once the wagons start rolling the friction, which is already low, goes down. The most difficult part would be to start the thing rolling. After reading your question I wondered if having a bit of slack in the couplings is actually intentional. This would mean the engine could start pulling the wagons very slowly and the front ones would start rolling before the back ones, which would again lower the friction.

Answer 10

Two words, my good man: Horsepower, and Experience.
Also, the couplings are just a tad slack so that the engine can pick 'em up one by one (in freight trains anyway). The engine picks up the first car, who's motion helps to start the second, the second helps to start the third, and so on. As to smooth metal wheels on smooth metal rails, you're not factoring in the great weight of the engine, and even if the wheels do start to slip, the engineer sprays sand onto the rails to increase traction.

Answer 11

I'm also finding this question interesting..
The website below explains everything..it is so complicated that it has to be explained thoroughly...without the brains and special trainings of those locomotive Engineers, the trains would not be able to pull the weight ... but as answerers above explained very well of its operation, anyway hope the links helps more..
..
LOCOMOTIVE:A self-propelled vehicle, usually electric or diesel-powered, for pulling or pushing freight or passenger cars on railroad tracks.
A driving or pulling force; an impetus: “The US could no longer serve as the locomotive for the world economy”

The locomotive is a wonderful machine. It performs tasks which thousands of men working together could not do. It pulls
heavy passenger and freight trains for long distances at great speed. But it would be helpless and useless without the locomotive engineer and his assistant, the fireman. They give it life and strength and direct its energies to the service of man. Under their direction it becomes one of our great and valuable servants.
It is very stricted to go on train tracks and dangerous, but I have seen and took pictures on one of the biggest freight train..just for remembrance...
Oh my! I'm on the 80th answerers..anyways.....
good day!