Though it was apparent to knowledgeable people in the railroad industry that reciprocating piston driven steam locomotives might soon be replaced by diesel power, Lima and C&O set out to build the ultimate in high power steam locomotives and they succeeded. No diesel engine ever surpassed the output of these monsters, which were the heaviest steam locomotives ever constructed (by engine-only weight). The 3 axle trailing truck supporting the firebox was unusual, carrying over 190,000 lbs, allowing the huge firebox needed for the high power. As it turned out, steam locomotives continued in service almost another 20 years.
Gene Huddleston's book, "C&O Power", reports tests of the C&O with a dynamometer car indicating readings between 6,700 to 6,900 hp at about 45 mph. The state of calibration of the dynamometer car is not known. The calculated starting tractive effort was only 110,200 lbf., but no one has published a higher dynamometer horsepower for any steam locomotive.
The locomotive was built to power coal trains on the 0.57% eastward climb from White Sulphur Springs, WV to Allegheny, VA. With one at the front and another at the back, 11500-ton coal trains left Hinton, WV and were at full throttle from White Sulphur Springs (a resort) to the top. C&O's 2-6-6-6s also handled coal trains from West Virginia to Columbus, Ohio. Huddleston says 23 locomotives were equipped with steam piping for heating passenger trains.
Multiple unit operation in the diesel era removed the need to pack the highest horsepower in a single unit. The H-8's 80000+ pound axle load demanded heavy rail and track structure.