W is the diameter of the driving wheels in inches and 85 percent accounts for typical operations with the steam cut-off valve at 85 percent. The cut-off valve effectively determines how much steam enters the cylinders on each stroke. When accelerating, steam locomotives typically were operated with no cut off, meaning cylinders would receive the full pressure of the boiler on each stroke. As the locomotive approached cruising speeds, the engineer would engage the cut-off valve, reducing the tractive effort but saving steam. Today's Diesels are typically rated in terms of horsepower.
Steam locomotives achieve their greatest horsepower at high speeds.
The Northern Pacific rated sisters to the 700 at 4,500 horsepower at 50 miles per hour and at 5,600 horsepower at 60 miles per hour.