When curves are designed for trains, sometimes the outer rail is elevated or banked, so that a locomotive can safely negotiate the curve at a higher speed. See the figure. Suppose a circular curve is designed for 60 miles per hour. The formula f(x) = 2540/x computes the elevation y in inches of the outer track for a curve having a radius of x feet, where y = f(x). 

(a) Evaluate f(400) and interpret the result. 

(b) Graph f in [0, 600, 100] by [0, 50, 5]. How does the elevation change as the radius increases? 

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