DIANA BETH SOLOMON and SHARAY ANGULO MEXICO will find out who was responsible for the overpass collapse that killed at least 23 people and injured dozens more when a train on Mexico City's newest metro line plunged onto a busy road below, the government has promised. Mexican President Andres Manuel Lopez Obrador said the investigation should be done quickly and that nothing should be hidden from the public. "There's no impunity for anyone," he told a news conference. The crash has raised questions about safety on one of the world's busiest metro systems, which spreads across an urban sprawl home…