I did a lot more planning and tweaking and deciding, and was finally able to get some MDF cut for the sides of the board, and the track bed.

After more tests I went with keeping things simple and not having any elevation changes on the track. I think the deciding factor was listening to the Z locos struggle on a 2% gradient - the Märklin Doppelstockwagen driving trailers have power pickups which add quite a lot of drag, and the hard-working motors are amplified by the lightweight board. I'm a bit sad about this since the prototype is heavily graded, but it's supposed to be an exhibition layout and reliability is more important than authenticity! At least for my sanity

Given that decision, it made sense to use the flat surface I'd built for the track datum, which meant cutting down the front for the valley. This was a bit nerve-wracking, since it involved partially un-making my nice square rigid board. It took revenge by stabbing me in the finger, naturally, but I prevailed in removing 50mm from the front rail, ends and interior bracing.

I've since built up the valley floor again, with the roadway:

which is starting to look about right. Re-covering the top has restored most of the strength, and the MDF profile boards should finish that job.

The MDF for the road and trackbed hasn't been fixed down yet - I want to cut some more out out the track bed to reduce the weight first. When your basic materials are so light even a small piece of 3mm MDF makes a big difference.

It seems like slow progress, but a lot of work has gone into this, most of it in my head, looking at pictures and watching videos to try and see what the terrain does in real life. The leading photo' of the bridge is to check that an angle from some pictures I saw on Panoramio works on the model. I probably should have an extra 10mm under the bridge to give some leeway, but I didn't want to reduce the longitudinal rail any further, so 50mm it is. I've already had to piece together a stream bed below the 50mm mark - you can just make out in the pictures where the pieces fit among the diagonal bracing.

The Spitfire once again proved its worth as a maid of all work - the largest piece in there is 1900x500mm - not small!

The last piece of good news is that an eBay parcel which was lost in the post arrived this morning after a month in transit :D