MBTA investigators ruled out mechanical failure Tuesday as a cause of a crash Monday in which a 60-foot T bus smashed into a trailer near the set of a Sandra Bullock movie being filmed near Dudley Square. The accident severely damaged the bus and sent about 15 people to hospitals with minor injuries. An inspection by Transit Police, the MBTA’s bus maintenance and safety departments, and the state Department of Public Utilities found no defects in the bus’s brakes, suspension, or steering systems. Police continue to investigate, and the driver remains out of service with the investigation ongoing. She took Tuesday off and will be assigned light duty if she returns before investigators reach a conclusion, MBTA spokesman Joe Pesaturo said.