OK, so now I just need to get the courage together to do this......
Pic 01 :
I started off by marking the position for the tunnel entrance and exit.
Then I made a cut so I do not damage the track or surrounding area.
Pic 02 :
Then very carefully I saw a section out of the facia and started to cut the foam away.
Pic 03 :
Tunnel is cut and the PVC pipe fits in nicely

Pic 04 :
I glued a section of styrofoam at the bottom of the layout to give the tunnel a base.
The layout is only built on two sheets of 20mm styrofoam, so cutting in under the roadbed will go right through.
Next step is to glue in the road, then fit the tunnel, then the entrance and exit and then finally paint, weather and landscape

So, I'll post again once I have made so more progress

Cheers,
Evert.