Several noteworthy curves are to be found on the railroads of the United States. The longest, where the
Illinois Central railroad rounds Lake Pontchartrain in Louisiana, is 9.45 miles. This is only a slight curve, but its
sharpness varies throughout its length. Another is the famous Horseshoe Curve of the Penn Central Railroad near
Altoona, which achieves its fame not from its length but from its shape. The curve is more than a semicircle and
is used to lengthen the line as the track climbs over the Allegheny Mountains. The grade on this curve is a steep
1.8 percent, but if a tangent were constructed connecting the lower and higher ends, its grade would be over 8
Most railway curves, however, are not as spectacular as these. They are less sharp and shorter, although
most are longer than 100 feet. And they are scarcely noticed by anyone except maintenance-of-way workers to
whom they represent extra work. Railway curves are quite numerous and represent approximately one-seventh of
all the rail trackage in the United States.
The distinctive characteristics of curved track and its highly technical layout and maintenance are vitally
important to track maintenance men. Consequently, the material in this chapter requires close and careful
reading. The chapter is divided into five sections: section I deals with the characteristics of curves; section II
discusses superelevation; section III explains spirals; section IV describes stringlining; and section V details the
lining of curves.
Section I. Characteristics
Railway curves are needed to change the direction of rail lines. Such changing is necessary to join
tangents, bypass obstructions, reach points not on the tangents, and gain elevation. This section describes the
general types of curves and their lining and measurement.