The 2023 Union Pacific AA units were the first early series Alco diesels produced by Lionel. These early Alcos -- produced from 1950 until 1954 -- are quality engines with many desired features. All early series Alcos remain in demand today. The 2023 Union Pacific ran for two years. For its first year 1950, the diesel was painted yellow with red lettering, quite similar to its real-life counterpart. This yellow version is frequently referred to as the Anniversary Diesels. For 1951, Lionel changed the color of the Union Pacific diesel to be a light gray diesel with a dark gray top.