More than 20 people are dead in Mexico City after a train crashed on an overpass. The MTA's trains run on 60 miles of overhead tracks, and the agency is reassuring riders, its lines are inspected regularly.

The MTA says crews inspect every mile of track twice a week, looking for any defects, broken ties, or bent rails -- things that can cause a derailment. And a spokesperson for the transit agency says structural engineers visually inspect elevated structures annually. They make lists of any damage, and prioritize what needs to be fixed.

A couple years ago construction debris began falling from elevated subway lines in Queens. The MTA eventually spent $325 million dollars to put netting under all elevated tracks in order to catch falling objects.