You are clearly in one of the dry areas, so the wood may survive longer.
The 2x4 lumber is used for two steps in the SNL process: 1) it provides the roadbed for straight sections and 2) it is cut into short 4” sections and used between the PVC trim on curves to keep the curves in line.
I do have one comment.You space the PVC quite widely on the curves and the blocks are far apart. Proper track design (especially ladder types) suggests you support the track under the rails, lengthwise, so your PVC strips should be 1.5"-2" apart. With the big gaps and wide spacing, there will be subsidence between the blocks and you will end up with a roller-coaster. Any heavy loco will cause the same effect.