Minor clarification: In Don's formula above, the '308' has been reduced a little bit - down to about 82 % - from the theoretical number of '375' to allow for friction and other mechanical losses and non-traction uses in the locomotive's machinery. If you derive the formula from scratch, it's as follows:
1 HP = 550 ft.-lbs./ second = 550 lbs. x 1 ft./ sec.
Since 1 MPH = 5,280 ft. / (60 mins. x 60 secs.) = 1.47 ft. /sec.,
Divide by the 1.47 ft. / sec. per 1 MPH = 550/ 1.47 = 374.15 - call it 375.
Thus, the theoretical formula is: HP x 375 = TE (in lbs.) X Speed (in MPH).
And 308 / 374.15 = 82.3 %.
Saying with Don's more realistic '308' value, the equation can be rearranged to solve for TE and Speed if the other 2 values are known, as follows:
TE (in lbs.) = HP x 308 / Speed (in MPH).
Speed (in MPH) = HP x 308 / TE (in lbs.)
Note that for a constant HP, Speed and TE are inverses of each other - as one rises, the other drops commensurately. That's a pretty important principle and characteristic of diesel locomotives to know and understand when discussing these matters.
- Paul North.
"This Fascinating Railroad Business"
(title of 1943 book by Robert Selph Henry of the AAR)