FORMULAS OF TRAIN RESISTANCES.
The work which a locomotive performs in pulling a train is expended in overcoming the resistances due to wheel-friction, gradients, curves, and atmospheric or wind pressure. Formulas have been propounded for calculating all train resistances, but they are utterly untrustworthy for American railroad trains. The best known formula of this kind is that given by D. K. Clark in his Railway Machinery. One calculation will show how misleading its figures are when applied to American railroad train resistances. Figured by the Clark formula, the total resistance per ton of a passenger train running at a speed of fifty miles an hour on a straight level track, is 22.6 pounds. By accurate records with his dynagraph car, Professor P. H. Dudley found the total resistances of an express train running at a speed of fifty-one miles an hour, to be 11 pounds. The resistances are so much different under different conditions, that nothing closer than a loose approximation can be calculated of the work done by a locomotive, unless indicator or dynamometer tests are made.