First baseboard built.
Underneath transverse bracing was fitted (6mm thick x 50mm deep) to stop any torsional twisting
The polyfoam is dense enough to take 4" x no12 screws which would grip sufficiently to allow the side/end panels to be screwed tight enough whilst the adhesive takes. This saves a lot of clamping up. The bonus with this type of construction is its lightness. Upper scenery, embankments, contours will be made from the conventional white polystyrene, the sort available from builders merchants in 50/100mm thickness 2440x1220mm size sheets as it's a lot cheaper.
The next one that will be made will built allowing for an 'under rail' type bridge. Not quite worked that out yet!
All in all, I'm very pleased with the final thing. I got the idea from an old issue of MRJ, which showed the construction of the baseboards for Gordon Gravett's 'Pempoul'.