Figure 3.3. Geometric Spiral and Circular and Spiraled Curves.
Spirals are used to ease trains into and out of curves and are thus correctly called easement curves. In
brief, that is how spirals are used, but more details are needed. As you read the following explanation, study the
diagram of the spiraled curve in figure 3.3, beginning with TS at the lower left; the pairs of letters identified here
also appear on the diagram. Straight track joins a spiral at a point where both are tangent--TS. Then the spiral
continues, getting sharper until it is as sharp as the circular curve--SC. From this point, the track follows the
circular curve to the place where another spiral begins--CS, from which point it becomes less sharp until it joins
the next tangent--ST.
Since the curvature of the spiral increases throughout its length, the superelevation of the outside rail can
be gradually increased from zero at the end of the tangent to the calculated maximum at the beginning of the
circular curve. On the spiral at the other end of the circular curve, the superelevation decreases from the amount
used in the circular curve to zero at the next tangent. The spiral therefore provides the proper amount of
superelevation for a train's speed without a sudden change in the banking.