Unlike stone or masonry retaining walls, retaining walls made of railroad ties do not weigh enough to retain soil without proper modifications. Installing deadmen in the retaining wall helps anchor it since the ends of the deadmen are anchored by the weight of the soil above the deadmen. Railroad ties rot from exposure to water, requiring you to replace the ties. Modifying the soil behind the retaining wall prevents water from accumulating behind the wall, forcing it to flow from behind the railroad ties so they do not rot.
Dig out the dirt sitting within 3 feet of the retaining wall using a shovel. Pile the dirt on a tarp so you can replace the dirt after you fix the retaining wall.
Remove any ties that are spongy, cracked or crumbling on any side. Measure the old ties and cut replacement ties the same length, using sawhorses and a handsaw.
Reposition the railroad ties so they sit in a slight step configuration, if they are stacked straight up, with an offset of 1 inch. The steps should climb toward the dirt the wall is retaining. Also, stagger the ends of the ties, like bricks, to further strengthen the wall.
Construct deadmen to help keep the wall from leaning later. Move the railroad ties in the retaining wall to insert cedar posts that measure 3 feet long and are the same thickness as the ties. The posts should extend into the area where the dirt was sitting.
Drive wood screws that are long enough to go through the ties and extend an additional 2 inches through the ends of the posts to secure posts of the same size that intersect the first posts.
Trim a drain tile using scissors. It should run the width of the wall plus 2 feet. Lay the tile within 3 inches of the retaining wall's backside. Move the bottom ties at one end of the wall and feed the drain tile's end through.
Spread a 6-inch layer of gravel over the drain tile, preventing soil from clogging the tile's openings.
Refill the dirt you removed, running a soil compactor over the soil after every 6 inches of soil you replace. Because you are compacting the soil, you will need to fill in extra soil until the soil's height sits even with the height of the wall, helping water drain over the wall instead of sitting behind it.