The first thing is to figure out the track plan that you want that will fit your location for the layout and how you want to operate the layout (turn diameter, one train or multiple, operating system, Toy, Hi rail, scale, one or more elevations, platform or open girder, etc.) You need to know this and have it all planned out before buying stuff. It may take a few days if you know what you want to several months if you have no set ideas or wants or know what others have done. Doing proper self examination as to what you like or want in train layout and proper planning will keep the regrets few in the future.
By the way, pine lumber should be fine as most houses are built from pine. You can get straight lumber at HD or maybe Lowes if you go though the boards and be picky. Most knots are not a problem. It may take a few trips a few weeks apart to pick out enough good straight boards. I have built 3, 8ft long shelves (and recently bought boards for a forth) for trains with 16 shelves of 1x4x 8ft pine and some 1x6 and 8x8ft for wide shelves and found straight boards.
I have written up in detail how I built my fifties era O27 operating and switching (27 of them) table layout with conventional control with 3 transformers, blocked to operate 5 trains at a time and homemade $10 turntable. If interested in something like this or to see how I went about mostly building it in a year or so and adding to it for 39 years, check it out on OGR link below.
https://ogrforum.ogaugerr.com/...ra-027-layout?page=1
Charlie
