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Thread: Friction physics in MSTS explanation?

  1. #11
    Join Date
    Nov 1999
    near Jacksonville, TX


    No, force does not give speed. The force to start a train moving (a diesel electric) (called starting tractive effort), is basically a function of adhesion and weight. Because of the way electric motors work, a locomotive with almost any power rating could start a train moving as a function of its weight and how well it grips the track. The electric motor is not happy about this condition for very long, so there is a second important constraint, continuous tractive effort, which is a function of a number of factors, the traction motors used, the transmission efficiency, the power available, among them. As you move down from full throttle to a lower notch, the continuous tractive effort will drop as well as power ( I am over simplifying here ). That force is measured at the minimum speed the locomotive can maintain at full throttle. To illustrate the difference in force and power, imagine hooking a 40 hp farm tractor to the back of a 600 HP Ferrari. If the tractor weighs 10,000 pounds and the Ferrari weighs 2,000 pounds, the tractor will slowly walk away pulling the Ferrari which will be wildly spinning the tires, but going backward. If it had 800 HP, it wouldn't do any better. Now the 600 HP Ferrari will easily out run a 40 HP tractor in a race, but it won't out pull it. Locomotives are the same way. You could use a very heavy SD9 on very heavy trains, but you won't be going very fast (roads like Mesabi and DMIR did just that, as they were not worried about priority traffic). Power is needed for speed. Most roads need to move the trains at 50-70 mph, so high horsepower diesels are needed to move long trains at track speed. The friction line is very important, as a light train with TOFC will have much greater air resistance than a heavier train of reefer boxcars, all alike.

  2. #12
    Join Date
    Jun 2013


    For a little reference, DMIR's SD9s were ballasted to 367,000lbs, and were geared such that they physically couldn't exceed 50mph. This gearing also gave them extreme pulling power at slow to moderate speeds. Not bad for 1,750hp

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