For the mid to late 1930s, wooden-sided boxcars with steel ends would probably be the most popular choices--you'd still find some all-wood boxcars with steel frames, and even some all-wood boxcars (with truss rods) on the rails, generally 36 or 40 footers. 36' wooden reefers were still standard...40' metal boxcars were in use by then, too. Gondolas were around, with metal sides, and open-topped hopper cars (no enclosed hoppers yet, grain and other loose items were packed in boxcars with wooden or cardboard walls blocking most of the door.) Roof walks were of course still in general use, and every freight train had a caboose.
Heavyweight (metal-sided) passenger cars were the rule, but the new streamlined cars were coming into use for high-profile fast passenger trains--the diesel-powered Pioneer Zephyr set the mold, the Super Chief and plenty of others followed with gleaming stainless-steel cars. Some electric interurbans still used wood-sided but metal-framed cars, although some had attempted to modernize by putting metal on top of the wood.
Tank cars, I think, ran 30-36 feet.