Complex sentence: Service began between Concord and MacArthur stations on May 21,1973 and the line was extended to Daly City station when the Transbay Tube opened on September 16,1974.
Split into simpler sentences: Service began between Concord and MacArthur stations on May 21,1973. The line was extended to Daly City station when the Transbay Tube was opened on September 16,1974.