The BNSF uses 6.2 horsepower per ton for calculating horsepower requirements on Tehachapi Pass in California. Tehachapi Pass has a ruling grade of 2.52%. Raton Pass has a ruling grade of 3.25%.
We have learned by experience operating trains up the grade on the layout that a typical HO scale diesel locomotive can only pull about 6ea. freight cars up the grade unassisted if the freight cars have been weighted to NMRA standards.
We wondered if the BNSF horsepower formula would be accurate on the model railroad as well. The switch lists used for our operating sessions provides the weight of every car used on the layout. We totaled up the prototype tonnage figures of every car on the layout and divided that figure by the number of cars to get an average weight in tons per car. The average weight came out to 59.1 tons per car.
Then we took the total horsepower of every diesel engine used on the layout and got an average horsepower rating for locomotives used on the layout. The average horsepower of locomotives used on the layout is 2,725.
Next, we took the 2.52% ruling grade figure from Tehachapi and divided it by 3.25 (Raton Pass) to get the difference in grade between Tehachapi and Raton. Raton Pass is 22.46 % steeper than Tehachapi. Multiplying Tehachapi’s horsepower rating of 6.2 by 22.46 percent gives us 7.59 for the horsepower rating for Raton Pass.
The average prototypical weight for 6ea. freight cars used on the layout (59.1 tons x 6) equals 354.6 tons. Therefore, to find the horsepower needed for six freight cars we multiplied 354.6 x 7.59, which gives us 2,691 horsepower, or one locomotive having an average horsepower of 2725.
- Average prototypical weight of freight car used on layout – 59.1 tons
- Raton Pass is 22.46% steeper than Tehachapi Pass. (2.52 divided by 3.25)
- Horsepower rating of Raton Pass is 7.59 tons per horsepower.
- Six freight cars at 59.1 tons each equal 354.6 tons. (59.1 x 354.6 = 2,691 HP)
- 2,691 HP is within one-percent of our average horsepower rating of 2,725.
- Diesel locomotives used for mainline trains on Santa Fe’s Raton Pass Model Railroad.
GP30 – 2,250 HP
GP35 – 2,500 HP
GP38 – 2,000 HP
SD40-2 3,000 HP
SD45 3,600 HP
C30-7 3,000 HP
16,350 HP divided by 6 = 2,725 average HP per locomotive.
It was quite remarkable to learn that the ability of HO scale locomotives to pull freight cars up the 3% grade on the layout, matched the calculations used by the real railroad within one-percent. It will be interesting to see if the same holds true for steam locomotives used on the layout.
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