The central concept is the use of a standalone Dighole component to punch a hole in the terrain grid through which spline objects representing track and tunnel bores can be passed. This allows for a much more flexible alignment of tunnelling but raises two issues in particular to overcome. Firstly the dighole will never be entirely filled by the tunnel portal and secondly the game camera will not automatically switch to an 'in-tunnel' view when a train enters.

The kit includes several components to mask the dighole, none of which are entirely satisfactory. Camera switching is dealt with by a script attached to the portals which takes control of the game camera, switching to cab view on entry and back on exit. This was my first script and it has its problems but it will be improved in time.