I use ordinary 3×3 fence posts for the supports, about four feet apart. The fence posts are inserted into the ground using Metposts, which take the form of a spike into which you can force the fence post, and there is also a bolt-down version for use on concrete. These make installing the supports a piece of cake.
The fence posts are joined on both sides by 4×1 timber. I asked the timber merchant for the longest they could manage, and they supplied 16-ft lengths. All timber (fence posts and 4×1) is tanalized (this is some kind of pressure treatment against the elements, it leaves a sort of green colouring on the surface of the wood) and I gave it a liberal coating of creosote, especially where I had cut the wood. I use four screws on each side where the 4×1 is joined to the fencepost: these are 2 inches long by No.10 BZP (bright zinc plated) twin-threaded woodscrews.
The track bed for the main line (phases 1 and 2) is made of ¾ inch thick exterior grade ply — this costs more than ordinary ply, but it is designed for wet conditions. For my four-track mainline the track bed is 1 foot wide, and for the six-track storage/staging yard it is 18 inches wide. The track bed is secured to the 4×1 supports with 1½ inch No.8 BZP woodscrews, about a foot apart, and staggered (i.e. fixed alternately to one support, then the other, then the first again, etc.).
On top of the trackbed is roofing felt, secured by bituminous adhesive, and overhanging the edge of the trackbed by at least 1 inch, preferably more as there is no point in letting too much weather get to the benchwork.
For the branch I tried experimenting with other techinques for building the baseboards. Basically, the boards are 150 mm. x 25 mm. planks of softwood, liberally soaked in preservative. If this doesn’t last the expected ten years, I shall probably replace it with plywood as used on the main line. The helix (q.v.) is done differently.