Complex sentence: Metrorail is an elevated rapid transit heavy rail system similar to a subway train that has one main line from Hialeah to Kendall and an under construction line known as the Airport Link to Miami International Airport.
Split into simpler sentences: Metrorail is an elevated rapid transit heavy rail system similar to a subway train. It has one main line from Hialeah to Kendall, and an extension known as the Airport Link to Miami International Airport.