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Originally Posted by KenWittlief
its more that that. The train wheels are welded to the same axle, so as the train goes around a bend the centripetal force causes the outside wheel to ride up on its inside (larger diameter) and the wheel on the inside of the curve is on the smaller part of its diameter. This allows the wheels to stay in contact with the rails without slipping, because the outside rail is longer than the inside rail on a curve.
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Just to finish off this train of thought (ha!), the AAR (Amercian Association of Railroads) standards established a wheel profile with a 3 degree slope across the width of the wheel tread. These standards were later adopted by the Federal Railway Administration. The fillet between the wheel tread and the flange used to be a simple circular radius. But in the past thirty years, as metallurgy has advanced and resulted in harder railhead materials, this simple profile caused uneven wheel wear and hollowing. In response, the fillet proifle is now a convoluted shape that minimizes wheel wear on the new, harder rails.
...and now back to our originally scheduled topic of discussion.
-dave