By | December 06 2012 3:03 PM

In November 2011, riders on Line 1 of the Paris Metro noticed a new feature on the 111-year-old subway line: platform screen doors, or PSDs. The transparent barriers separate subway platforms from the tracks. The doors are aligned with train doors and open simultaneously after the train has stopped. It’s a feature that serves multiple purposes, not the least of which is preventing riders from falling onto the tracks and being killed by trains.