I would recommend Atlas Snap-Track on the curved portions of your layout. On a 4' x 8' board, let us say you have an oval of track (NOT saying you have to do a oval, but for continuous running you will wind up with some sort of at least semi-circles at each end). If you use 22" radius curves, you will have about 2 inches on each side from the center of the track to the edge of the layout. That could present a hazard if a train gets knocked over the edge. Eighteen-inch radius Snap-Track will give you more room from the edge. Of course, you might combine pieces of 22" and 18" track, depending on the design. The pieces will fit together with no problem. There is also 15" radius Snap-Track, which might be used for sidings inside the wider radius curves. For the straight portions or moderately curved track, flex-track can be handy. It will depend on what rolling stock you want to use. I would definitely go with NS, as it is far easier to keep clean for good contact. (OTOH, I have used brass when I first started in the hobby, about 60 years ago, and salvaged and am using some on my current layout in yards and sidings. Just have to clean it once in a while.) You will probably want to stick with four-axle diesel locomotives and smaller steamers. Six-axle diesels can be problematic on 18" radius, but it will depend on the model.
Some steam locos can work fine on 18" curves. I would not recommend a 4-8-8-4 Big Boy, however! Probably a 2-8-2 Mikado would be about the largest. Some of the newer Bachmann engines are short enough wheel base for your purposes. I have kitbashed the out-of-production Mantua engines so that I can get 2-10-4's to run around 18-20" radius curves, but that takes some doing!
For freight cars, I would recommend 40-ft., although 50-ft will work. Passenger cars are more of a problem nowadays. Although you can get 85-ft cars to work on 18" curves, you would be better off finding some of the old Athearn Blue Box or ConCor shorties (72'), if you can find them.
I would use cork roadbed under your track.
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Stay well and safe!