A spiral or transition curve should be used in main-line tracks, if practicable, between tangent and curve
or between the different degrees of curvature of a compound curve. A spiral is also desirable in all tracks
other than main tracks, where practicable, between tangent and curves and between the different
degrees of curvature of a compound curve. The form of the spiral should be such that the degree of
curvature increases directly with the length. In other than main tracks with no superelevation, this
permits the engine or car trucks to be directed gradually to their rotated position (required by a curve)
rather than to be directed instantaneously. This will likewise tend to prevent distortion of the alignment
of the main body of the curve due to the force required to produce angular acceleration. In main tracks
with superelevation, this permits a uniform rate of change of elevation of the outer rail on the spiral and
promotes best riding conditions by maintaining throughout the train passage from tangent to curve the
desired relation between amount of superelevation and degree of curvature.