You have two: Static friction (prior to the start of it rolling) and Rolling friction (after you overcome static friction). Generally, you can use dry steel on dry steel, which, if I recall correctly, is about 0.3. After it starts rolling, I think it drops off to around 0.1. There is a railway engineering forum on eng-tips, I think. You may way to find it.
Way back when, there were articles in the ASME transactions on the "hunting" of railcars [bopping side-to-side]I seem to recall that empty cars had more friction because the wheel flanges rubbed more.
Under steady state conditions, train resistance can be approximated by the modified Davis equation, listed below.
I don't understand what you mean by using a counter-weight to haul a car, but if it is what I picture in my mind, then you have a constant force, which means acceleration. Note that most of the terms of the equation have a speed component.
Train Resistance:
TR = 1.3W + 29n + 0.045WV + [0.0024 + 0.00034(Q-1)]AV^2 pounds force where:
TR = Total train resistance in pounds force
W = Total train weight in tons
V = Train speed in miles per hour
A = Frontal area in square feet
Q = Number of cars in the train
n = Number of axles in the train
or TR = 6.4M + 130n + 0.14MS + [0.046 + 0.0065(Q-1)]AmS^2
Newtons where:
TR = Total train resistance in Newtons
M = Total train mass in metric tonnes
S = Train speed in kilometers per hour
Am = Frontal area in square meters
Q = Number of cars in the train
n = Number of axles in the train
Gbor, actually wheels don't work like that. Your calculation would be fine if the brakes were on!
For those of us who think in terms of rolling resistance, then a steel tire rolling on a steel rail has a coefficient of .001 to .002 according to the blue book, but MintJulep's equation is better.
The implied coefficient there is 6.4/(1000 * 9.81), 0.00065
Cheers
Greg Locock
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I think the greatest resistance will come from the type of bearings installed. I have seen box cars being pulled by a strong man when wheel bearings were equipped. Are all box car wheels now bearing equipped?
