Compensation For Curvature
Other things being equal, a train is harder to pull around a curve than it is on straight track because the wagons - especially bogie (2 axle) wagons - follow the chord of the curve and not the arc of the curve. To compensate for this, the gradient should be a little less steep the sharper the curve is; the necessary grade reduction is assumed to be given by a simple formula such as 0.04 per cent per "degree of curve", the latter being a measure of curve sharpness used in the United States. On a 10-degree curve (radius 573.7 feet) the grade would thus need to be 0.4% less than the grade on straight track.
In addition, the friction of the wheels against the curved rails increases the pull needed from the locomotive.
Read more about this topic: Ruling Gradient
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