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Working Of Additional Engine In Freight Trains

  1. Jan 12, 2010 #1
    1. I have heard the arguement that "the Effort ( Force ) required to move a certain mass by Pulling is greater than the Effort required to move the same mass by Pushing it".

    What is the truth regarding this statement and what causes the difference in the applied force to move the same magnitude of mass if at all there exsists a difference.

    2. Secondly I have always seen that there is an Additional Engine attatched in the middle of a very long train ( esp. goods/freight carriers with large number of wagons in comparison to passennger carriers) or for trains with steep uphill routes.

    a. I want to know that How exactly does this combination work and help in the easier motion of the train ie how does the Pull of the front engine and Push of the second engine act on the wagons and provide the force togehter.

    b. How exactly does the Pull of the front engine affect the second engine and the push of the second engine affect the motion of the front engine and the wagons ahead of itself.

    c. Is there any interrelation which may affect the forces exerted by each of them or do these push of one and pull of other influence the resultant force exerted by the combination.
    ( because it seems that the additon of the second engine causes a reduction in the effort which would otherwise be exerted if a single engine of the total HP of the combination was used).
  2. jcsd
  3. Jan 12, 2010 #2

    Ranger Mike

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    is it harder to push a chain than pull it?
  4. Jan 12, 2010 #3
    Yep, Mike , thats what I've read ( rather explained by a person on a platform for engineering disussions) in one place.

    And I'm in a dilemma about the correctness of the statement. ( I personally dont see any difference between the two, but this is just to confirm it and see if it actually is the case).

    A more technical view from you would be a better reply, so if you can please share your view.
  5. Jan 12, 2010 #4


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    If you fill each link with something solid, it would make a better anology.
    I think a sewer cleaning cable might be more in line with the train, if you push it in the pipe from too far back it will tend to flex before moving into the pipe, if you use two hands spread the proper distance, the flex point is more stabel and a greater force can be applied.

    Just my .02
  6. Jan 14, 2010 #5

    WHy are you guys divertung the topic? And from where did the chain come in picture? I've not spoken about chains at all.

    I am clearly mentioning about a simple mass ( say a block) kept on the ground ( so friction included) is said to be easily pushed than Pulled to move it forward.

    The question is why here is such a difference in the amount of force required to move the block with the same mass and same coefficient of friction by just changing the method( ie pushing it or pulling it)

    The second question is a set of three actually, which are quite clear.

    So can please anyone enlighten us regarding the above query. And yes, with relevant responses.

  7. Jan 14, 2010 #6
    It might be easier for a human being to push rather then pull but it makes no difference to an engine.
    As to putting the extra engine in the middle of a train think about the forces on couplings between wagons, if both engines are at the front all the force has to be transmitted through the first wagon a bit less through the second and so on, by putting your extra engine in the middle you reduce the load on the couplings.
  8. Jan 14, 2010 #7
    the main reason trains will use an added engine in the middle, or at the ends it not really a function of easier or harder. The chain is an important analogy because a train is flexible and must bend to go around corners. You can put all the power up front, but if there are sharp corners and steep grades, the vector of the force tends to pull the cars to the inside of the corner. Locomotives in the middle or at the end reduce this tendency as the train is for all practicle purposes reduced in length, and the force is "smoothed out" thru the length. The pulls of both engines will be, as far as able to control, evenly split between all engines, regardless of location in the train

  9. Jan 14, 2010 #8


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    I thought my anology of two hands on a flexable cable was very relevant.

    As jobrag stated, each coupleing between cars represents a flex point and all RR tracks have dips, humps and curves. The power applied at different points and gravity determine how the cars remain in contact with the tracks.

    Why is there a limit to number of cars in a train ? the first one I think of is how long can a crossing be blocked.
  10. Jan 14, 2010 #9
    train length is limited by the passing sidings, grades, and radius of corners more than how long a road can be blocked.

  11. Jan 14, 2010 #10
    Thing is, the bogies (set of wheels) twist, it makes no difference to the engine if the tracks are bent or straight.

    Jobrag is right, if each car in a train has an engine and each wagon is therefore working, makes things easier, the strain on the links between them is smaller. If on the other hand you have one supper engine at the front/back, there would be a lot of strain on the wagon's link as this one is pulling all the weight of the train. Trains are not light too.

    Reason number two, easier for more engines to move thousands of tonnes.
  12. Jan 14, 2010 #11
    the engine is pulling in a straight line. engine and train enters a 90 degree corner. the vector of force is actually 90 degrees to the cars after the corner. That means the engine will try to pull the cars at the 90 degree angle, and the force must be "bent" via the cars making the corner. the wheels only allow it to go around the corner

    Last edited: Jan 14, 2010
  13. Jan 14, 2010 #12
    Correct. In addition, as a long train (the longest trains are now about 10,000 feet long) goes over a hill, initially the front engine is pulling and the rear engine is pushing. As more and more of the train goes over the hill, the front engine gradually reduces its pulling and begins pushing back on the train and the rear engine also reverses and begins pulling back.

    The critical factor determining how long a train can be is the strength of the knuckle that connects the cars. For a long, heavy train going up a hill, the stress on a knuckle can be tremendous. A locomotive in the middle reduces that stress.
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