[BAracer]

Thanks for the responses. I like the idea of the expanding foam. I might try and get it done in one shift to avoid the tube cleaning!

The existing track pieces seem to be stuck to the ply base with silicon. I think I will continue with that method once I settle on the layout, and give the track pieces a good clean.

I was thinking of building up the ground level either side of the track pieces with the right thickness foamboard to give a flush finish either side of the track. I am imagining the expanded foam will be the wrong product in the wrong location for that. Any objections to the use of foamboard, or any better ideas?