Use pressure-treated plywood to cover the wall. Place a row of 1x6s along the bottom of the railroad-tie retaining wall so the plywood doesn't sit directly on the patio. Press a sheet of pressure-treated plywood against the wall, resting on the 1x6s, and attach it to the wall with exterior-grade screws. Trim the plywood flush with the top of the wall. Repeat this process to cover the entire front of the wall with plywood. After the 1x6s are removed, the space at the bottom of the wall will help water drain from behind the wall if necessary.