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.
Use a staple gun to cover the plywood with felt paper, wrapping the felt paper over the top of the plywood and railroad ties. Keep the felt paper snug against the wall and overlap the sheets. Use roofing nails to attach a layer of diamond mesh over the felt paper; this mesh will provide a structure for the mortar and stone to stick to. Space the nails about 18 inches apart to hold the mesh tight against the wall.
Mix mortar according to the package instructions, either using a mortar mixer or a heavy-duty drill with a paddle bit. Use a wide trowel to apply an even coat of mortar over the mesh, then press the faux stones into the mortar. Nail wall ties (L-shaped brackets) to the wall at the mortar joints to help support the stones. Arrange the stones so they fit evenly. Use a random pattern for the stones, making sure the joints between stones don't line up.Wet each stone before placing it to keep it from sucking all the moisture out of the mortar.
At the top of the wall, spread mortar and press in cap stones. Butter the edges of the cap stones with mortar to create a mortar joint between the stones as well as below them. The cap stones in this project are beveled to help with drainage.