\chapter{Transportation}

      The transportation process is responsible for determining the
geometrical limits of a step.  It calculates the length of step with which a
track will cross into 
another volume.  When the track actually arrives at a boundary, the
transportation process locates the next volume that it enters.

   If the particle is charged and there is an electromagnetic (or
potentially other) field, it is responsible for propagating the particle in
this field.  It does this according to an equation of motion.  This equation 
can be provided by Geant4, for the case a magnetic or EM field, 
or can be provided by the user for other fields.

   The transportation updates the time of flight of a particle, utilising its
initial velocity.

\textit{Some additional details on motion in fields:}

  In order to intersect the model Geant4 geometry of a detector or setup, the
curved trajectory followed by a charged particle is split into 'chords segments'.  A
chord is a straight line segment between two trajectory points.  Chords are
created utilizing a criterion for the maximum estimated distance between a
curve point and the chord. This distance is also known as the sagitta.

  The equations of motions are solved utilising Runge Kutta methods.
Runge Kutta methods of different can be utilised for fields depending on the
numerical method utilised for approximating the field.  Specialised methods
for near-constant magnetic fields are under development.