200 series freights are on 12.5" frames, introduced in 1926 and made through late 1930's. 500 series freights were introduced 1927, made through late 1930's, are on 11.5" frames. 200's were more expensive and came in sets with larger locos, 500's less expensive, came with smaller locos.
A book that would be very helpful to you is the TCA book "Lionel Trains, Standard of the World". It covers all production from 1900 2-7/8" through 1943, including Standard, O, OO, paper train and accessories. The book shows up on ebay often. The book is well-organized and comprehensive.
