constructed in theaters are not to include spirals. However, they are not to be disregarded entirely, because there
may be some already built and in operation.
The spiral curve differs from the circular curve in that the curvature of the spiral varies uniformly
throughout its length. It is used to connect tangent track and a circular curve so that a train moves from tangent
track through a spiral into the circular curve, through another spiral, and on to tangent track. Because of the great
length of time required to properly construct spirals, military engineers have specified that spirals are not to be
included in constructing curves in a theater when the degree of curvature is 2 degrees or less.
Section IV. Stringlining
The requirements for well-maintained curved track are the same as those for straight track, but many
maintenance jobs are more difficult to perform on curves, and some faults of curved track must be corrected more
frequently than the same ones on tangents. For instance, rails wear faster on curves. And while both curves and
tangents require proper lining, you can line a straight stretch of track by eye but must calculate corrections in
detail to line a curve properly.
Although the design and construction of curved track are intended to counterbalance the forces applied by
trains rounding curves, they can never compensate for them entirely. Sooner or later the continual passage of
trains results in track movement, and the lining is disturbed. First, the ride becomes uncomfortable and the rails
suffer great wear; eventually, the curve becomes unsafe for scheduled speeds. Constant watchfulness by
maintenance men, however, can prevent this happening. Curves are checked frequently, and when one becomes
distorted, it is lined promptly to avoid further damage.
Engineers use surveying equipment to lay out railway curves; in the past they were lined in the same way.
Now, however, they are almost always lined by a comparatively simple method known as stringlining. When
poor riding qualities or visual inspections indicate that a curve is poorly lined, a stringlining job is required.