MEXICO CITY, June 28 (Reuters) - Mexico City Mayor Claudia Sheinbaum on Monday said a veteran transportation official will take over as director of the city"s sprawling Metro system. Guillermo Calderon begins the new role on Monday, Sheinbaum told a news conference. "He is an honest public servant who will now be in charge of the metro public transport system," said Sheinbaum. Calderon will replace Florencia Serrania, who was at the helm of the Metro when part of the Line 12 collapsed in May, killing 26 people. The preliminary results of an ongoing investigation showed that the incident was triggered by structural problems. Sheinbaum did not explain the reason for Serrania"s replacement.