I've a number of 1 and 2mm sheets of ply left over from a previous hobby/project which I intend to make use of here.

Here is the start of the construction of the tunnel by making a ply armature which will be overlaid with Modroc.

Work on the track will have to wait a few more weeks as I will have to wait for C&L to get back to normal after their break-in and robbery last weekend.