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Freight train is more efficient than truck due to lower rolling resistance. And I wonder which one has lower rolling resistance<>, small diameter or larger one or it doesn't not depend on diameter at all? Both are steel wheel on steel rail.

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Have you tried searching for it? Since wikipedia has a page. – fibonatic Jul 10 '13 at 12:00
up vote 5 down vote accepted

If you look at the Wikipeda article on rolling resistance that @fibonatic pointed out, you can find an equation for the coefficient of rolling friction:

$C_{rr}=\sqrt {\frac z d}$

There is another equation after it that is for steel on steel, but that also shows that C is proportional to $d^{-\frac 1 2}$. So from this, you can easily see that a smaller diameter will have a higher C, and thus a higher resistance. A freight train will have a larger diameter than a truck, meaning less resistance, thus being more efficient.

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