In the early Lionel catalogs O27 was a marketing term for "cheaper" sets. The engine was often exactly the same as the "O" version, but with different cab numbers: the K4 O27 was 2025, the O was 675. The price difference for the engine and tender alone was 50 cents...$24.50 vs $25.! Set-wise, the O27 came with a stipped-down caboose (no lighting and less details). The Turbine was similar. The Berkshire only came in O. I assume it was too long for O27 curves.
Today many new trains need a curve wider than O31...some MUCH wider. Even O36 is now minimum for many new trains.
