To get the ballast right up to the platform edges in a nice neat straight line, I needed to figure out how to apply it.  Because of the overhanging lip of the platforms it was not possible to accurately sprinkle the sand or add glue mixture in the traditional way.

I opted to make some templates from card:

I marked these with a centre line and fixed them in place using Copydex glue, then went about applying ballast in the tried and tested way.

Once dried (overnight) I trimmed along the edges of the templates and went about removing them.  The Copydex worked a bit too well in places and some of the card stayed behind, also some of the ballast inevitably lifted with the templates...

The platform sections fit quite neatly into the spaces though -as the edges of the ballast are quite flexible.  I only have a couple of gaps to fill in now, so for the most part I think it 

worked well.