Platform screen doors are best described as

• A. They are decorative barriers along platform sides.
• B. They are doors that block all access to the tracks permanently.
• C. Doors installed at the platform edge that align with train doors to prevent falls and improve climate control and air quality.
• D. They are sensors that detect passenger density on the platform.

Answer: (C)

Platform screen doors are safety barriers installed at the edge of a train platform that align with the train doors so they open and close in sync with trains. This creates a controlled boundary between the platform and the tracks, preventing passengers from accidentally falling onto the tracks and deterring access to the rail area. Because the doors seal the gap around the train doors, they also help with climate control by reducing drafts and air exchange between the tunnel and the station, which can improve air quality. They are not merely decorative barriers, they do not permanently block access to the tracks, and they are not sensors for passenger density.