Can you get treated railroad ties. The real ones, not the garden variety. What they call switch ties, the ones for the crossings are longer. I had a load of sand set down, set the ties on that then added the framing of 24" centers. I used rough cut so they were bigger than kiln dried 2x4's. The walls are 8' high plus some extra height from the ties. The frames were lag bolted to the ties. I used proper rafters or trusses (I'm not up on the terminology) because of the snow load they have to withstand.