— Tim, Frenchtown, NJ
A: Tom Silva replies: It's probably okay to leave the ties in place — they sure won't rot. Just cut out the rotten trim and box in the ties with medium-density overlay (MDO), an exterior-grade plywood faced with a phenolic-resin paper. MDO is stronger than standard plywood, takes paint very well, and isn't bothered by wet weather.
Nail ¾-inch MDO to the ties with 8d stainless steel finish nails, which won't corrode. Secure all the joints with a marine-grade polyurethane adhesive and the same stainless nails. Marine adhesive is waterproof, holds like iron, and helps seal joints against water penetration.
While we're on this topic, let me warn anyone who's thinking about using creosote-soaked railroad ties for their home or garden to think again. Creosote smells bad, gets on everything, contaminates soil and water, and probably causes cancer. You don't want to touch it, cut it, or have it exposed anywhere around the house, especially when safer pressure-treated timbers are available.