I have made sandwiches of 3ply, 40mm foam 3mm ply, which becomes incredibly strong, light and warp resistant. Standing my 150lb weight on a 3' 6" x 3' one, produces a deflection of just 1mm. It is also tolerant of shapes being cut out from above or below, to leave just a single layer of the ply - with a certain amount of common sense about how much and where...
Just to get the idea, these are just the droppers on a Shunting Puzzle, to which have been added, servos, polarity changing for the frogs, lighting etc. The small tunnel in the top right corner of the channel, is for wiring from the NCE control panel.
The 3' 6" x 3' board is to be used for an N Gauge run-around for some Berlin Military Train stuff and will have wiring, for Kato Unitrack run in grooves, Routed into the top ply. Those channels will have thin balsa strip laid in, or maybe Plastruct channelling to keep the wiring safe. Given the topic, the "handover of travel documents", hiding the channel under platforms, pavements etc, will require a minimum in the way of planning, marking and Routing.
That said, my, storable, larger layout, is made of 6mm ply, as per wartime a/c and Laser-cut by Tim Horne. The reason is simple - joining the boards together is perfectly possible with both formats, but far easier to manufacture in the ply boards, where the Patternmaker's Dowels and securing bolts also make for connections for 2 x Bus routes, one DCC and the other, DC.
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