I've made some progress on the track sections . . .
Picture of engine number one pulling 2 trucks along a finished piece of track. The last truck has had the original moulded ore removed and a floor added so that it can be used to carry troops or other items

Details of the track construction for those that are interested . . .
Following on from the previous work, the edges of the mdf trackbed were painted in a 50/50 mix of pva glue and emulsion (the same brown as they had already been painted). Patches of small stones were added and then it was all covered in coarse sand. It was then sprayed with a fine mist of water from a plant spray bottle - this dampens the sand and draws the pva into it to give a very strong bond when dry.

The sand and small stones came from a bag of builder's sand bought for £1.50 from B&Q. I sieved a quantity using a cheap metal sieve and separated out the larger grit to use as ballast. The alternative would be to use model railway ballast but my way's a lot cheaper
Once the PVA/paint mix was dry I repainted the edges in the base brown coat and then applied two lighter shades of drybrushing

Next came the ballasting. Working with the track piece flat on the workbench, I added the larger grit between the sleepers with an old teaspoon, brushing them out evenly using a dry old paintbrush - no glue was added at this stage. Once all the gaps between the speakers were filled I then sprayed it with water from the spray bottle to get the stones wet.

Using a craft syringe I added a few drops of a 50/50 PVA/Water mix to each pile of stones. The piece was then set aside to dry thoroughly. Once dry, the process was repeated to ballast the outside of the track. I decided to do the ballasting in two parts mainly because of the difficulty of handling the track piece with so many loose stones that would be on the piece if I did it in one go.

Once the ballast grit had all dried I applied patches of static grass and clumps of foliage with the 50/50 PVA/Water mix, again giving the finished piece a light misting of water to help draw the pva into the grass and give it a strong bond. One thing worth mentioning is that consideration needs to be given to the ends of each track piece to avoid unnatural looking joins. If one piece is flocked and another isn't, an odd looking straight line will result. I've decided not to apply any flock to the ends of the pieces to avoid this.
Hope you find this useful.
Next on the agenda, a pair of flat bed trucks . . .