The RER lines (Réseau Express Régional or Regional Express Network) are different from other train lines because they do not terminate in Paris. They cross the capital underground, so that passengers can get from one side of the region to the other on the same line.

The Paris RER is a regional train transport system connecting the center to its surrounding suburbs. RER stands for Réseau Express Régional. The RER also complements the Paris Metro, giving Parisians and tourists a larger transport network in the centre of Paris.