compensation for such a wide differential in speed; restricting the speed of the faster trains is then necessary.
Military engineers specify that 4 inches of superelevation is the maximum to be used. If this amount of
superelevation is not enough for the speeds expected, speed limits have to be imposed. Again refer to table IV; it
shows permissible speeds at various super-elevations when the degree of curvature is as shown in the left column
of that table.
Raising the outside rail of a curved track vertically is known as superelevating the track. Superelevation
counteracts the lateral thrust and centrifugal force exerted by locomotives and rolling stock. The amount of
superelevation depends on the kind of curves, the sharpness of the curve, the gage of track, and the speed of
trains. On a military railroad, the maximum amount of superelevation recommended for any curve is 4 inches.
Section III. Spirals
The history of railroading is marked by a continuous increase in train speeds. Gradually, it became
evident that mere arcs of circles could not serve satisfactorily as complete curves. Locomotives traveling at
increased speeds exerted great force on the outside rail of a curve at the point where the locomotive entered the
curve from the tangent, and struck another blow to the next tangent on leaving the curve. Since it was impossible
to superelevate the entire curve without disturbing the level surface of the tangents at either end, speed had to be
reduced while traveling around the curve, passengers were subjected to discomfort, and an element of danger was
introduced. Including a spiral, or easement curve, between the tangent and the circular curve eliminated the
A spiral, although a curve, differs from a circular, or simple, curve in that the curvature as well as the
radii of the spiral vary uniformly throughout its length. Starting from the open end of the spiral, the curvature
becomes sharper toward the closed end. A simple geometric spiral, a circular curve, and a spiraled curve are
shown in figure 3.3. The way in which spirals are used and designed is explained in the next two paragraphs.