Complex sentence: Trains running northbound (toward Main Street) normally open their doors on the island platform; the northbound side platform is used only during Mets games and events at the National Tennis Center, such as the U.S. Open.
Split into simpler sentences: Trains running northbound (toward Main Street) normally open their doors on the island platform. The northbound side platform is used only during Mets games and events at the National Tennis Center, such as the U.S. Open.