Complex sentence: The 1939 World's Fair was served by GG trains, some of which were marked as S Special, via the short lived IND World's Fair Line to Horace Harding Boulevard.
Split into simpler sentences: The 1939 World's Fair was served by GG trains, some of which were marked as S Special. The trains ran via the short-lived IND World's Fair Line to Horace Harding Boulevard.