This is the basic
boxfile with track placed in to get an idea of how things will work. I
have cut some 3mm thick MDF to shape and allowed slots for wiring and
point operation.

Pins soldered through the strip will work micro switches to change the polarity of the point frogs. If you used Peco points, this wouldn't be necessary as they sort out their own electrics.
The first train runs!
The fiddle yard is
another file with an MDF base. Track has been extended over. Power is
transferred through the bolts that hold the "boards" together.
The flap is held up with a wooden dowel to hide the contents and maintain the sense of mystery.
At
the other end, the track extends into an extension tube. This can be packed
in the fiddle yard when not required - as allowed for in the rules.
Close up of the extension
tube made of the remains of the MDF I lined the bases with. I've had to
lop a bit off the top to get it to fit in the fiddle yard.
Thin MDF cuts nicely with a Stanley knife and steel ruler, a bit like cardboard modeling. Mind you, more cuts are required but the result is stronger. PVA glue joins are braced with stripwood as didn't want anything coming apart in transit.
You can clearly see the bolts holding the "boards" together which also transfer track power. Cheapo DIY ones were used which turned out to be a mistake as the wires refuse to stay soldered to them. Not a problem in the yards but inside the buildings, access is very limited for repairs. I'd recommend sourcing some proper brass fittings if you are trying this.

The track has been sprayed with primer and then track-colour from Precision paints aerosol. Card inserts are being stuck in to bring the ground up to the level of the top of the sleepers. Ballasting experiments have also begun. The top one uses traditional sawdust with watered down PVA. The bottom uses the same stuff but the glue was painted onto the card/sleepers. Tradition has won out here !