My method for covering the bits that I don't like is to cover them up with concrete. To do this I make sure my track bed is supported at the right levels ext and then cover this with old bits of chicken wire. This is then covered with sacking cut into strips. I bought a roll of Brick layers Hessian, the sort they use to cover brick work in the winter. When these strips are soaked in a very runny mix of soft sand and cement they are laid over the wire and moulded to a shape that suites and left to dry for a couple of days. This is then followed at a later stage with a mix of grit sand and cement applied by trowel, witch hopefully looks more realistic. This can be painted at a latter stage or just left to collect natures own colours. As here