Railroad Miles. The greatest combined length of U.S.-owned operating railroad track existed in 1916, when industrial activity increased during World War I. The total length has decreased ever since. The data over the years 1900 to 2008 are modeled by the quartic function
f(x) = - 0.004091x4 + 1.275179x3
- 142.589291x2 + 5069.1067x
+ 197,909.1675,
Where x is the number of years since 1900 and f (x) is in miles (Association of American Railroads). Find the number of miles of operating railroad track in the United States in 1916, in 1960, and in 1985, and estimate the number in 2010. (The lengths exclude yard tracks, sidings, and parallel tracks.)