Some parts of the layout have curtains on the sides allowing quick access to hidden track areas below, but in other areas where there are no trains below, I typically make the side out of some plywood.
Curtains are more suitable in places where the edge of the layout is at a similar height. When topography results in different heights, plywood, is better.
Here a river valley is defined by the plywood fascia.
and was painted dark green once the landscaping was done.
Curtains and side fascias provide a very 'solid' feel to the landscape.