Two responses: 1) I prepare my logos for shrouds much like Chris described above, only I use MS Publisher. I make a "grid" using the line draw tool that has loads the right height and length, but some are offset slightly as wold be a real load. I download and crop logos off the internet. The best results I have gotten are from photos of actual lumber loads. In larger scales you can even get the wrinkle effect printed right onto the logo. I crop and size the logos, then I place them on the load grid, making sure they don't all line up across different stacks and some are partial as they would wrap around the ends of stacks. I have not modeled the stacks separately thus far thinking it was unnecessary in N scale, but seeing Chris' photos here I may try doing so and see how they turn out.
2) Railcoat2, Christ is right, for the time period you are modeling I think the loads would be unwrapped and lumber would be hauled on regular flatcars, maybe bulkheads, and better lumber in boxcars. I don't believe center beam flatcars had come into use yet in the time period you are thinking.
Ron