In equation (4), the first term is the flexural strainenergy in the rail, and the second term denotes theenergy in the track springs. Equation (4) can be castinto the standard smoothing spline problem asmin 1 p ð Þ ðlEId2yr dx2 2dx + pXi yr xi ð Þ yb xi ð Þ ½ 2 8