For my first trestle, I made each individual bent to fit the uneven topography beneath the trestle, but I also stubbed the ends on buried brick foundations. After a few years, the ends rotted but worse, I had glued all the pieces with Elmer’s outdoor glue, so that eventually everything fell apart. What a mess. So I rebuilt the trestle and made a second, even longer one for another part of the layout. I made just 2-3 sizes of bents, nailed everything with brass escutcheon pins, and propped the bents upon bricks ABOVE ground level, and I adjusted the height of the bricks to compensate for the fixed height of the bent. Now I have two strong trestles that I don’t expect to rot in my lifetime. As far as drainage from one side to the other, they - and I - put in a culvert when needed.