In all the years I planned the movements for NKP 765, I never ran across the term "steam impact" as it relates to bridges.
All bridges have a "Cooper Rating" or it may also be called an "E-Rating." These ratings are determined by the length of the bridge span and other construction details of the bridge. The equipment that will pass over that bridge will demand a certain E-rating, depending on that equipment's weight and wheelbase, in other words, exactly where the weight is concentrated.
Whenever the 765 was to operate on a new railroad, I would send them a package of info on the locomotive. That package included several pages of Bridge Loading Analysis done for us in 1983 by Chris Berger, who was then VP of Operations at the Chicago & Northwestern Railroad. The "Complete Mechanical Package" as I called it, also included a Clearance Diagram, so the host railroad could determine if there were any places on the railroad where clearances would not allow the engine to go.
Here are a few pages from that package:
The Cover Page
The Clearance Diagram
One of the FIVE Bridge Loading Analysis pages. This one is for the full engine consist. There are other loading sheets for the locomotive only, the tender only, the loco and tender, the auxiliary tender and the full consist of loco, tender and A-tank.
The Union Pacific has a similar package of information on the 4014 that they provide other railroads where they have trackage rights and will want to move the locomotive.
