The RER, or Réseau Express Régional, is a rapid transit system serving the Île-de-France region, which includes Paris and its suburbs. It consists of several lines that connect the city to surrounding areas, making it easier for commuters to travel quickly and efficiently. The RER operates both underground and above ground, providing access to key destinations, including major train stations and airports. The RER is divided into five main lines, labeled A through E, each identified by a letter. It is an essential part of the public transportation network, complementing the Métro system and providing a vital link for millions of passengers daily.