Nearly 30% of the buses across the system did not arrive on time May 9, the most recent date for which data was available on the MTA’s “Performance Improvement” page as of Thursday. The buses can be tracked on the agency’s Transit app with real-time GPS devices, a service that will expand to the MTA’s MARC commuter trains and light rail in July and the Baltimore Metro Subway in the late fall, MTA Administrator Kevin Quinn said.