Paris is served by a network of commuter express trains, running on five different routes within the city and its suburbs. The Reseau Express Regional (RER) connects the city's underground metro system with its suburban commuter raili links. The RER offers a number of interchanges with the metro system and major stations, but offers a service with fewer stops, thereby acting as more of an express system.
In total, the RER network includes 246 stations, with nearly 600km of track. Its five lines are simply named Line A to Line E.
If you are travelling on the RER within Paris' Zone 1 area, you can use a standard metro ticket for your journey. Beyond this area, passengers require a Billet Ile de France. Ticket prices vary depending on the distance travelled (for journeys outside Zone 1), and tickets can be bought from any RER or metro station. Longer length passes are also available, for travel over the day, number of days or week.