In terms of mechanical efficiency, is it more efficient for a freight train to push it's cars or pull them?

Question

Please answer in terms of physics, not money or time savings. I realize that sometimes freight trains may push or pull their cargo simply because the engine is located in that position and it would take too much effort to relocate it.

Answer 1

There is no difference. The engine is with the engineer is always in front of the traveling load, controlling speed for conditions and watching for trouble in the path.

Answer 2

that depends, are you talking in the daytime or at night.

Answer 3

I am certainly no physicist and I have poor grades to prove it.  I do have some time behind the throttle under my belt, however. But, the physics of rolling train resistance is expressed thusly:  Horsepower per ton, times 12, divided by the percentage of the grade will equal the speed a given train will travel on the grade.  Simply put, HPT x 12 / %G = S.

As a practical matter, an engine can push more tonnage than it can pull, if for no other reason than in "draft", you are limited to the stress that the draft gear can take before failing.  When pushing, this is no longer part of the equation.  However, now in "buff", jackknifing becomes a real possibility, so there are definite limits there.

As far as control is concerned, pushing is better that pulling.  Often, in both road and local freight operations, the cars are routinely pulled or pushed with a light brake application on each car.  In local freight and work train serving, you're pushing them around all day long with the air in them.  This is especially true in work train service, as it is usually precise work, dumping ballast or unloading ribbon rail, or when out picking up scrap iron from along the right of way.

Taking the above into account, again as a practical matter from the mechanical stand point, it is in fact more eficient to push rather than to pull.  As far as the physics, gravtity is the same evreywhere on the planet and is both friend and foe.

Very good question.  Hope this does it for ya.

Answer 4

Consider these factors:

Efficiency is the ratio of output work to input effort.   Or ton-mile moved vs. fuel used.

The starting effort to move a train by pushing or pulling should be the same because it is a matter of the locomotive tractive and adhesive forces overcoming the static friction of the first car, sustaining momentum and pulling (or pulling) each successive car in the train.  There should be no difference between pulling or pushing to start a train.

Once the train is moving, the effort required to maintain motion is a matter of overcoming any frontal wind, total friction on the entire train's axles, and any air friction in between cars.  The most efficient configuration would be for the leading vehicle - locomotive or freight car - with the most aerodynamic frontal shape.  An advantage might be given to a streamlined locomotive compared to the flat front of a boxcar.

However, for a long train, there are at least two benefits of the locomotive in front:  greater crew forward visibility to the terrain and any obstructions on the tracks;   and to use the engine's brakes plus compression of all trains slack on the locomotive's drawbar  to safely brake or stop the train in a hurry.

A long train being pushed from the rear could easily suffer a coupler break in two if the locomotive in the rear tried to apply aggressive braking  while the slack was 'in' (the cars were all compressed together.)  If the loco in rear were to brake hard, the coupler slack would unwind from the rear and the train could stretch hard & quickly to break a weak coupler.  Then the forward part of the train becomes a runaway.

Answer 5

pulling has more control, than pushing

Answer 6

Back to your original question, in terms of mechanical efficiency, none.