Understanding the PRPA Logo

700/SP&S logo

PRPA's logo consists of the SP&S oval intertwined with the lettering on the cab of the 700. Beneath the locomotive number, railroads often lettered basic information about the size and weight of the locomotive. This was useful for dispatchers who needed to know how much freight a locomotive could pull. It was also useful for engine crews, who were sometimes paid extra for operating larger locomotives.

The information beneath the 700 includes:

Tractive effort = B^2 * S * BP * 85% / W where: 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. But this was wasteful because much of the power would be lost when the steam exhausted (which is what makes steam locomotives "chuff"). 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.