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

"I think I can, I think can, I think can"

" I thought I could, I thought I could, I thought I could!"

WHOO HOO!

(I love Dumbo) lol

Answer 2

train fairies?

Answer 3

I think it has alot to do with the power from the engine.

Answer 4

lol i love trains

well the train is already so powerful and strong and its engine is probably bigger than the train (almost) and the train is at its highest speed so its pulling all of these cargo's

but if it went the same speed as it did with the cargo, it would go straight off the tracks without the cargo because of how fast it goes

don't forget, the train is on wheels, so then its easier to pull such a huge, strong force, because if it did not have wheels, it would go nowhere, thats why the strong men contests always pull stuff on wheels

Answer 5

i'm not familiar with the fancy words used in other answers but my answer is that once the train gets going, the cars just follow without the engine using that much power.

Answer 6

Each wagon's traction force (force required to be applied against gravity to "move" the weight from point A to point B) is resolved by four wheels at each corner of the wagon but it still has to be pulled with a "greater traction force" that pulls all the wagons in engine form . The engine is the leader of the train on the path to God (point B). Thats why the engine needs more power derived from fuel the fuel gives energy when it destroys or ceases to exist itself (coal), same way, God is using all life forms when they die God uses them to derive energy for his train (which is part of his bigger game design plan)...

Answer 7

i think it it the way a train is constructed and how well....if u make a tree house, and u dont nail it together, it will fall....if you put it together firmly, it can withsatnd your weight

Answer 8

diesel engines, plain and simple...I think I can, I think I can, has nothing to do with it

Answer 9

it is torque to horse power ratio

Answer 10

The power and strenght pulls the wieght

Answer 11

Lots of horsepower.

Answer 12

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 13

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 14

A NICKEL!

Answer 15

That smooth metal is the key.  The stiffness and low coefficient of friction creates low drag.  Think of car tires.  They are filled to about 30-40psi air.  Big trucks, about 100psi.  The steel wheels on a train are even stiffer.  They just don't deform very much.

Answer 16

Cause they have a lot of energy and they are bigger than the loads and the centrufigal force forces it forward.

Or it takes steroids(chuckle)

Answer 17

the key element in my opinion are the "wheels" -- the round wheels -- if the wheels had another format -- such as square or triangle etc. --this whole process of pulling specially in a train with so much weight could not have happened. we owe it all to the ingenuity of the round wheels and their effect.

Answer 18

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 19

the friction between the steel wheels and the track is less

Answer 20

Hoghead, your explanation sounds right to me.  I kept waiting for you the mention the sand under the drive wheels for traction and, well, you did mentioned.   Good explanation.  Pops

Answer 21

we live in the 21st century

Answer 22

Torque and momentum

Answer 23

Locos can attain a maximum speed of 250 Km/hr. But our tracks are not designed for such a high speed. Or i can also say that tracks are designed for 300 Km/hr, but that is for standard conditions. Indian railways take in account a safety factor of 50%. That is why they say that the tracks are designed for 150 Km/hr.

The power produced in Locos is variable. The locos is run by traction motors. There are four traction motors. One traction motor produces a power of 1200 BHP. If all the motors are running they produce a power of 4800 BHP. The running of the motors depends on the power required to pull the wagons

Answer 24

It all has to do with the torque. The trains have an incredible amount of weight, but there are numerous cars, rather than one large one. This reduces the weight the train has to pull. Also, once a train gets to its desired speed, the law that states "an object in motion stays in motion" kicks in. The train keeps on going because of its intertia. This is why it takes so long for a train to stop; its inertia is so great that it simply wants to keep on going.

Answer 25

it's because they usually weigh more than what they're pulling with high horsepower and on smooth track's

Answer 26

Maybe it very big, I haven't  seen a train yet.

Answer 27

its because they start so slowly... they start their speed up slowly... and the momentum keeps them going.. its similar to a rollercoaster... the initial fall keeps it going thruout the ride... newtons law 'object in motion stays in motion unless another force is acted upon it'. thats why trains cant stop fast... simple physics

Answer 28

First of all u should remember that   wheel on any smooth surface rolling nice.with this man decided to add more strength to pull more loads  we use steam pressure,diesel,gas,to pull faster the secret beyond the pulling power of heavy loads by trains is man who found it.the secret beyond all these invention and discoveries is GOD.

Answer 29

very slowly

Answer 30

The metal is smooth, but with all of that weight, it deforms some, and there is adhesion between wheel and track. Diesel engines of 6000 hp run generators that drive DC electric motors to turn the wheels, giving better control than a mechanical transmission.

Answer 31

i not thing to be mean but thay have alot of mexicans pulling them