Even though this is just a test track, I still feel it should be ballasted. Looking through various books it wasn't easy to find photos of industrial track as opposed to the bucolic country lines. I suppose that the temporary nature of these little railways combined with staff not interested in the demands of the camera toting enthusiasts mean less pictures for us modellers to gawp at.
Those that I did find didn't show much. Real lines were granite ballasted and the texture shows up. Industrial concerns seemed just to be smooth. At a guess, the main constituent of trackbed was mud and dirt. How to replicate this is another problem. Even the articles in Narrow Gauge and Industrial didn't seem much help as the subject was glossed over.
My attempts started with sawdust but this didn't seem to have any texture at all when prodded into place. Next up I considered real ash (we have a coal fire) but aside from the supply being a bit light in these warm spring days, it's horrible stuff to work with - one of the reasons steam vanished from BR tracks !
Digging around in my scenic supply I found some dried tea leaves. In this case the contents of teabags emptied out onto a tray and dried in the oven. Thinking "Nothing ventured, nothing gained", I sprinkled and prodded these into place and then applied dilute PVA with a dash of washing up liquid via a pipette.
The result looks worryingly too much like a bed of leaves. I think I need a bit of plaster or sawdust to take away some of the texture and then some paint as the tea is a bit brown.