The Réseau Express Régional (French pronunciation: [ʁezo ɛkspʁɛs ʁeʒjɔnal]; English: Regional Express Network), commonly abbreviated RER (pronounced [ɛʁəɛʁ]), is a hybrid commuter rail and rapid transit system, similar to the S-Bahns of German-speaking countries and the S Lines of Milan, serving Paris and its suburbs.

The RER was not fully conceptualised until the completion of the Schéma directeur d'aménagement et d'urbanisme (roughly: "master plan for urban development") in 1965.

The RER network, which initially comprised two lines, was formally inaugurated on 8 December 1977 in a ceremony that was attended by President Valéry Giscard d'Estaing.

[9] In comparison to the Metro, the RER provides better coverage of Paris' suburbs and typically operates at higher speeds and with greater distances between stations.

The company's post-war successor, RATP, revived the scheme in the 1950s, and in 1960 an interministerial committee decided to go ahead with the construction of an east-west line.

[12] Subsequently, the central part of the RER was completed between 1962 and 1977 in a large-scale civil engineering project whose chief supervisor was Siavash Teimouri.

The construction of the RER was a major undertaking, being highly visible to both Parisians and visiting tourists at various sites across the city for an extended period.

The embryonic (and as yet unnamed) RER was not properly conceived until the 1965 Schéma directeur d'aménagement et d'urbanisme (roughly: "master plan for urban development"), which envisioned an H-shaped network with two north-south routes.

[citation needed] Only a single north-south route crossing the Left Bank has so far come to fruition, although the Métro's line 13 has been extended to perform a similar function.

[16] This initial investment, along with subsequent spending, was partly financed by the versement transport, a local tax levied on businesses that was introduced in July 1971.

SNCF gained the authorisation to operate its own routes, which became lines C, D and E. Extensive sections of suburban tracks were added to the network, but only four new stations were built.

[22] No new building work was necessary at Châtelet – Les Halles, as additional platforms for Line D had been built at the time of the station's construction 20 years earlier.

On the RER, interconnection required the development of specific trains (MI 79 series for Materiel d'Interconnexion 1979, and MI 2N series for Materiel d'Interconnexion à 2 niveaux (double-deck interconnection stock)) capable of operating under both 1.5 kV direct current on the RATP network and 25 kV / 50 Hz alternating current on the SNCF network.

This is due to a 1961 decision to build them according to a loading gauge standard created by the Union Internationale des Chemins de Fer (UIC), with space for overhead catenary power supply to trains.

In 2017, it was announced that an consortium comprising Alstom and Bombardier Transportation had been selected to supply 255 X’Trapolis Cityduplex double-deck electric multiple units to replace aging rolling stock on both lines D and E under a €3.75 billion arrangement.

Using the model of the existing Métro, and unlike any other underground network in the world, engineers elected to build the three new deep stations (Étoile, Auber and Nation) as single monolithic halls with lateral platforms and no supporting pillars.

By bringing far-flung suburbs within easy reach of central Paris, the network has aided the reintegration of the traditionally insular capital with its periphery.

[30] The RER has received criticism for its high level of particle pollution during busy periods, largely due to train braking.

Pollution by PM10 particles regularly reaches 400 μg/m3 at Auber,[31] much more than at neighboring metro stations and eight times the EU Commission's daily average limit [32] of 50 μg/m3.