Re: UP Freight Conductors' Train Books

Tim O'Connor

Mike, I thought diesel horsepower was a function of the RPM's
of the diesel engine... Diesel hp is independent of the loco's
mph, while steam hp does indeed increase with mph. I think you
may be confusing horsepower with tractive effort. The fact that
a 4-6-6-4 could produce almost 6,000 hp or a 4-8-8-4 7,000 hp
should surprise no one. But as you note, that horsepower was
only available in a limited speed range, while diesels provided
constant hp from 10 to 60 mph.

And sometimes diesel engines were shipped on flat cars... :-)

Tim O'Connor

Is this finding related to the random data you posted, or are you
referring to some other documentation?
Union Pacific Motive Power in Transition 1936-1960, by Lloyd Stagner, pg 45.
Actually, I wrote from memory. The exact quote is: "Pull/speed graphs
indicated that at speeds above 25 mph, the 4000 class [ Big Boy's ] and 4
GP-9's were equal, that the 3900's exceeded 3 GP-9's at all speeds above 22
mph and that both were outperforming turbines at speeds above 17 mph."
Obviously at speeds above 25 mph, Big Boys were superior to a 3 unit F unit
and I slighted the 3900 by giving it equal rating with a 3 unit diesel. Mind
you, we are talking about speeds above about 25 mph. At lower speeds, where
the max horsepower of a diesel was available, the diesel ruled. What this is has been proven out many times empirically, high horsepower
steam locomotives generally were designed to produce their max HP at
relatively higher speeds. Thus, the two UP engines referred to achieved max
HP in the 40-50 mph range whereas the diesel achieved its at 20 mph. See the
plots on pg 83 of N&W Giant of Steam. Incidentally, the plots were of a 6000
HP 4 unit diesel which was later sold to...uh...UP.

Mike Brock

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