Complex sentence: Gresley conjugated valve gear was used by the American Locomotive Company under license and the 4-12-2 locomotives they built for the Union Pacific Railroad between 1926 and 1930 were the largest locomotives to use this valve gear.
Split into simpler sentences: Gresley conjugated valve gear was used by the American Locomotive Company under license for the 4-12-2 locomotives built for the Union Pacific Railroad between 1926 and 1930. These were the largest locomotives to use this valve gear.