On main tracks, it is preferred to establish the transition from tangent level track and
curved superelevated track by the use of a spiral or easement curve. A spiral is a curve
whose degree of curve varies exponentially from infinity (tangent) to the degree of the
body curve. The spiral completes two functions, including the gradual introduction of
superelevation as well as guiding the railway vehicle from tangent track to curved track.
Without it, there would be very high lateral dynamic load acting on the first portion of
the curve and the first portion of tangent past the curve due to the sudden introduction
and removal of centrifugal forces associated with the body curve.