The Réseau Express Régional (RER) is a rapid transit system serving the Île-de-France region, which includes the city of Paris. It connects the city center with its suburbs, providing efficient transportation for commuters and travelers. The RER consists of five main lines, labeled A through E, and operates both underground and above ground. The RER is integrated with the Paris Métro and other regional transport services, making it easy for passengers to transfer between different modes of transport. It plays a crucial role in reducing traffic congestion and promoting public transport use in the densely populated area surrounding Paris.