Hi Phil, thanks for giving me some things to consider.
Firstly, the ply thickness. The basic baseboard is already finished, using 9mm birch ply on 22 x 47 PAR perimeter framing. The upper level hasn't been built yet, but the plan is to use 6mm birch ply on PAR spacer blocks. This will be cantilevered over the lower tracks and the blocks positioned to suit the layout (not the other way around). In order to maximise the headroom over the gradient track I can use 3mm or 4mm ply (or maybe even cardboard come to that) for a narrow area over the lower track only.
Now to your questions:
A Two levels:
1 The layout is on two levels in order to increase the number of separate tracks (and therefore operators) from 2 to 3. Also it gives a more definite break between urban and rural scenery.
2 Very good point about the hidden trackwork, it made me think carefully. The furthest hidden track is only about 100mm from the baseboard edge, so it shouldn't be too difficult to rescue any derailed trains. The loop nearest the edge is intended to be the re-railing point for all stock on the complete layout. However, I'm going to see whether it's feasible to make the country station a removable section for lower track cleaning and access.
3 Originally the layout was designed to be controlled from the main line station side. This was before it was decided where the layout would go in the school, and when it transpired there is very little space, and that the layout needed to be either portable or hinged against a wall, I had to reverse it round in order to retain access to the hidden loops. You're right it will hide the main line station area to an extent, but the difference in levels is only 45mm so I don't think it's a major problem. I envisaged the main line station on a bridge over the tracks so it will be more visible, but the kids will decide that.
B: Wall mounted and hinged:
1 There is going to be a 6mm ply surround about 175mm high fixed to the edge of the baseboard on all four sides (with an opening to access the hidden loops). The ply will be flush with the underside of the PAR framing and project above the layout about 125mm, so it forms an open box. This will be deep enough to protect all the scenery, and also prevent anything falling on the floor. The surround stiffens the baseboard and stops it twisting and flexing, and I envisage it will be painted to form a backdrop.
2 The surround will be sufficiently tough to support strap hinges bolted through it. I think 4 hinges will be OK. It might need stiffening along the top edge and I have some aluminium angle to screw on if necessary. When it's hinged up it will be secured to the wall, probably with a padlock and chain!
3 Yes it's going to be a PITA to take down and set up. If only there was sufficient space for a permanent layout. Hinging it is better than completely portable though.
4 There will always be at least 2 adults on hand to help raise and lower the board. The bare baseboard as it stands is surprisingly light - around 15kg.