derek sawyer who provide a time loob line of exactly what happened. take a listen. >> 9:3. fire personnel responded encountered six overturned trains. we declare it a mass casualty event. at 9:35. we struck out the third alarm. then at 10:24 we struck the fourth alarm. as the mayor said that brings 33 apparatus, 122 personnel. it is a level three mass casualty incident means 18 medic units brought on scene. we have a total of six critical transports to temple university, einstein, tarsdale, and a total of 43 transports, buses, medic units for less critical injuries. others are walking wounded. >> derek sawyer there. former "the new york times" safety expert matthew wald said