For ballasting I use sandpit sand bought from ToysRUs before it disappeared. It is reasonably coarse so can pass for ballast in n gauge. 

One of the things I do not like about commercial N gauge track is the depth of the sleepers. I therefore wanted to use the ballast to hide this depth consistently. After some experimentation I came up with a scheme to achieve this.

Once the track was laid, I lined the edge of the sleepers with evergreen 1mm quarter-round StripStyrene. The depth is just a little lower than Peco code 55 sleeper depth. An example of this can be seen in the attached image showing a bit of test track.

Ballast was then glued to the edge strip using water proof wood working adhesive painted on to the strip with a fine brush. Note that the water proof wood working adhesive was used for this step to avoid the next step, which involves water, undoing this step. Once this was dry the remaining gaps between the sleepers were ballasted in the traditional way by spreading ballast carefully and setting with watered down PVA with a little washing up liquid to kill surface tension. This creates the illusion of a ballast shoulder on which the sleepers are resting. 

It was then airbrush weathered using a range of colours but mostly Precision Paints Track Colour.

EE D6827 has no real business being there. I just like it!