jms, You can replace the lower wooden rail without taking the top off the trailer. I am in the process of replacing them in my 1999 21' Towlite and have the curbside long 16' one done already. Just support the top on all 4 corners with wooden blocks down to the frame and then tear out the rotten plywood from the bottom side of the top wall. I then ripped a new piece from a pressure treated 4x4 post and pushed back up into place. I then screwed through the steel tubing frame from the outside with 2 1/2" screws to hold the wood in place, placing a screw every foot along the side. After that, I put pre-painted metal flashing along the side that came up 2" on the outside and bent under to cover the wooden beam. Now I have a solid, continuous support beam the full length of that side instead of plywood. I intend to do the same to the remainder of the walls as well.