camping mat subroadbed

I don't know the name of the plastic, but the rolls of 12mm dense foam sold as camping sleeping mats make great sub-roadbed. it deadens sound, provides a firm base, is waterprooof, dimensionally stable, curves easily, cuts easily...
i cut the rolls into strips using a large kitehen knife and a steel straight-edge. i cut one side at a 45-degree angle to give ditch shoulders, and the other side square, so I end up with the same design as cork roadbed only on a larger scale. this helps follow a centre-line and it curves more easily.

Double-track main has two 30mm square-sided strips between the shouldered strips.

I glue it down to plywood with latex caluk or PVA.

It works beautifully with a "Surform" rasp, though nearly as messy as styrofoam.

Shop around carefully. In New Zealand there is a great variation in density of the foam (and some variation in thickness) between brands - I prefer the denser stuff. Maybe the softer stuff would work just as well but instinct tells me I want the firmer base. I also managed to find a few charcoal coloured mats so more lurid colours won't peek out through the sscenery

tent mats
In large areas such as my staging yard, i found 600mm square floor mats for tents at KMart made from the same stuff only tougher and denser than any sleep mats i've seen. they dove-tail together. OK they have holes punched in them but I'll be laying cork sheet over them anyway so i don;'t think it'll be a problem, and they were even cheaper per sq metre than the sleep mats.