I am using 1/4" hardboard running underneath the elevated portion of the layout. I have made my own elevated trestles on the portion of the layout that shows the most. There are 1" strips of heavy cardboard attached to the trestles, covering the sides of the 1/4" hardboard. The cardboard is painted gray to simulate steel plate girders. I think this gives a more finished look to the elevated section. I’ll try to post a detailed photo if you are interested.