Re: Calculating Motive Power Requirements


Prototype methods and basis for locomotive ratings and assignments appear to vary widely by railroad, division, and era.

I would strongly urge checking into your prototype's methods for the location and era of interest. I have not investigated widely, but several have researched the PRR's steam era practice and it was pretty sophisticated, especially on divisions where stalled trains could not be tolerated due to the backup and delay of other trains. I have not found any evidence that dispatchers had any input to locomotive selection on the PRR - I believe it was enforced by traffic managers and road foreman.

For each division and terminal pair the PRR would provide a tonnage rating for each class of locomotive, with further breakdowns for priority freights versus "mineral" extras. Some of the PRR's published books even report the location of the ruling grade and the minimum speed that the RR expected trains to sustain on that grade.

The PRR did have a "de-rating" scheme for low temperatures and wet rail, but not, as far as I know, for the wind. But I have read on the OpSig group that modern railroads crossing the midwest have found wind direction and speed to play havoc with the ratings on double-stacks (obviously past our era) so they do have correction factors for those conditions. Conversely, with modern roller bearings, the impact of temperature on train drag is now much less.

For the prototype, the total elevation change of the grade can be important, since the prototype "enjoys" the benefits of trading momentum for changes in elevation. For the prototype, the momentum at 60 miles an hour can be traded for a climb of over 100 feet.

Not so for the modeler, where, in HO, the momentum of 60 scale mph translates to a climb of only 3/8 of a real inch. Even worse in N-Scale (no way to scale gravity). This is why it is so important to avoid short lengths of steeper than ruling grade on model railroads. A long 2% grade with a few feet of 2.5% grade will actually have a ruling grade of 2.5%, not 2%.

Conversely the PRR had several 25 foot high flyovers on their main line where starting the flyover at track speed would knock less than 20 mph off the train's speed, without changing the throttle. Such flyovers could actually have pretty steep grades, since the momentum effect doesn't care about the grade, only the change in elevation. Unfortunately this effect is not available to modelers, although, based on recent news reports, an G scale layout on Pluto might have close to G Scale gravity, but not HO - Pluto is still much too big for HO scale gravity ;-)

For HO model railroads, in general steam locomotives can never match their prototype ratings if pulling cars weighted to the NMRA RP, while diesels can often pull more than their prototype cousins. Within the PRR modeling community, Bowser steam locomotives do have a reputation for breaking that general statement, while their Kato based F-units significantly outperform the prototypes..

I would note I have a good friend who has outstanding track work, and was weighted most of his fleet to only 2/3 to 3/4 of NMRA RP, yet still has very reliable operations. That, combined with very free rolling trucks, has enabled his HO steam locomotives to come closer to prototype performance. (Remember that a large prototype steam locomotive on a 2% grade might be rated at less than twenty 50 ton (capacity) cars)

And there has been debate over the years on the LDSig group concerning curve compensation and grades. Best to test model equipment when designing a layout, although for diesels, there is little to worry about. Steam is the exact opposite, as those who try to run long steam trains on steep grades usually find out. Lots of discussion on this topic in the past on the LDSig and OpSig Yahoo groups, which I would recommend for those designing STMFC era layouts, especially those who plan to run steam on the head-end.

Dave Evans

---In STMFC@..., <jcdworkingonthenp@...> wrote :

I agree with Bruce on his statements regarding the employees timetables.  I believe that the dispatcher has the final say on signing off on the power assigned.  The wind and temperature are also a factor. The dispatchers train sheets have blanks for this data that is typically filled in.

Nelson, for more data look for the website/blogsite Tales from the Krug


It has some interesting data however out of respect for the sheriff, I will note the date frame of the site well post dates this sites 1960 cut off. Al Krug is an interesting railroader and has put some useful data on the site.  
                                                                                                       Jim Dick - St. Paul, MN

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