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It seems to me that for as difficult as it is to understand this subject, the greater difficulty is communicating it with words in a discussion forum.  I have some understanding of this subject, but I am also learning things as these threads progress.  So I would like to lay out some thoughts for review and to check my understanding.I have heard the term "tractive effort" many times over the years, but apparently have not correctly understood it.  I always interpreted it to mean drawbar pull.  As I think about it now, I don't recall if drawbar pull is a standard term of measure, or not.  In any case, my interpretation of drawbar pull is the actual pulling force of the locomotive exerted at the pulling coupler.  It would not only depend upon the pulling force that locomotive could apply to the wheels, but also to the traction of the wheels on the rails.  That traction is what I would call adhesion.  It is affected by the weight on the drivers and by the slipperiness of the rail/wheel contact.  As I understand it now, according to Fred, tractive effort does not include the affect of adhesion, so it is not drawbar pull.  Instead, tractive effort is pounds of force developed at the running tread of the drive wheels.  It is not affected by slipperiness of the rail or the weight on the drivers.  In fact locomotive tractive effort could be measured without even having a track.  I guess the expression of tractive effort would be the total of the drive wheel force of all drivers.  So a B-B diesel producing 100,000 pounds of TE would be exerting 12,500 pounds per wheel, at the running tread of each wheel.In order to compare the fundamental abilities of steam and diesel locomotives, we need to compare locomotives of equal stature for a fair comparison.  To determine if two locomotives are of equal stature, we could consider their attributes of horsepower rating, the actual production of horsepower at different speeds, tractive effort at various speeds, adhesion according to weight on drivers, number of drivers, and total locomotive weight. But here is the problem:  With so many variables in performance, no two locomotives will be identical unless they are the same model.  In fact, in our comparison of fundamental abilities, we expect to find a difference.  Yet if a difference is found, it implies that the two locomotives are not of equal stature, and therefore the comparison is unfair.  It strikes me that this is the reason that the question, which was the most powerful steam locomotive? has never been answered. So to at least simplify the task of comparing apples-to-apples in this discussion, I suggest we set aside the issue of adhesion.  I realize that the adhesion must be included to determine the ability of a locomotive, but it seems to me that neither locomotive type has any fundamental advantage in weight-on-driver determined adhesion if the other locomotive could be ballasted for the same result.  So why not just compare tractive effort at different speeds?If we do that to find the fundamental difference in tractive effort, what criteria do we use to match locomotives of equal stature, so the comparison is fair?
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