Editing Réseau Express Régional (section)

==Rolling stock==
The predominance of suburban SNCF track on the RER network explains why RER trains use [[overhead line]] power and run on the left, like SNCF trains (except in [[Alsace-Moselle]]), contrary to the [[Paris Métro|Métro]] where trains use [[third rail]] power and run on the right. RER trains run by the two different operators share the same track infrastructure, a practice called interconnection. On the RER, interconnection required the development of specific trains ([[MI 79]] series for {{lang|fr|Materiel d'Interconnexion}} 1979, and [[MI 2N]] series for {{lang|fr|Materiel d'Interconnexion à 2 niveaux}} (double-deck interconnection stock)) capable of operating under both 1.5 kV direct current on the [[RATP Group|RATP]] network and 25 kV / 50&nbsp;Hz alternating current on the SNCF network. The [[MS 61]] series ({{lang|fr|Matériel Suburbain 1961}}) can be used only on the 1.5 kV DC network.

The RER's tunnels have unusually large cross-sections. This is due to a 1961 decision to build them according to a [[loading gauge]] standard created by the {{lang|fr|[[Union Internationale des Chemins de Fer]]}} (UIC), with space for overhead [[overhead lines|catenary]] power supply to trains. Single-track tunnels measure {{convert|6.30|m|abbr=on}} across and double-track tunnels up to {{convert|8.70|m|abbr=on}}, meaning a cross-sectional area of up to {{convert|50|m2}}, larger than that of the {{lang|fr|stations}} on many comparable underground rail networks.{{r|cg2003|p=29}}

The first RER rolling stock in fact predated the formation of the RER by 40 years, with the [[Z 23000]] stock used on the {{lang|fr|[[ligne de Sceaux]]}} (which was thereafter integrated into [[RER B]]) from 1937 until 27 February 1987. In 1965 the [[SNCF Class Z 5300|Z 5300]] train was introduced, followed by the [[MS 61]] in 1967, [[MI 79]] in 1980, [[MI 84]] and [[SNCF Class Z 8800|Z 8800]] in 1985, [[SNCF Class Z 20500|Z 20500]] in 1988, [[MI 2N]] in 1996, [[SNCF Class Z 20900|Z 20900]] in 2001, [[MI 09]] on 5 December 2011, [[SNCF Class Z 50000|Z 50000]] (Francilien) in 2015 and [[Regio 2N]] (Z 57000) in 2019. In 2017, it was announced that an consortium comprising [[Alstom]] and [[Bombardier Transportation]] had been selected to supply 255 [[RER NG|X’Trapolis Cityduplex]] [[Bilevel rail car|double-deck]] [[electric multiple unit]]s to replace aging rolling stock on both lines D and E under a €3.75 billion arrangement.<ref>{{cite web |url = https://www.globalrailwayreview.com/news/31211/sncf-ile-de-france/ |title = SNCF selects consortium to supply 255 trains for Île-de-France network |first = Katie |last = Sadler |publisher = globalrailwayreview.com |date = 11 January 2017}}</ref> In April 2023, 60 additional Cityduplexs were ordered to increase service frequency.<ref>{{cite web |url = https://www.railadvent.co.uk/2023/04/alstom-to-supply-sixty-more-new-generation-trains-for-ile-de-france.html |title = Alstom to supply sixty more new-generation trains for Île-de-France |publisher = railadvent.co.uk |first = Janine |last = Booth |date = 27 April 2023}}</ref>

Many services are performed by double-decker train sets, usually operating in double formations.

<gallery class="center" widths="150">
File:Z-22901-idfm-versailles-chantiers.jpg|[[SNCF Class Z 20900|Z 20900]] at Versailles Chantiers ([[RER C]])
File:Z 57000 (Regio 2N) IDF Mobilités Ligne R.jpg|[[Regio 2N|Z 57000]] at Paris Gare de Lyon ([[Transilien Line R]] and [[RER D]])
File:Z50000 IDFM et SNCF Carmillon.jpg|[[SNCF Class Z 50000|Z 50000]] ([[RER E]] and [[Transilien Line P]])
File:Ligne de Nanterre a Sartrouville - Viaduc de Nanterre - MI09 - Avril 2012 (4) cropped.jpg|[[MI 09]] ([[RER A]])
File:MI79-R 8292 1.jpg|[[MI 79]] renovated ([[RER B]])
File:Gare de Corbeil-Essonnes - 2021-07-08 - IMG 7373.jpg|[[Z 20500]] at Corbeil-Essonnes ([[RER D]])
File:Gare de Vauboyen - 2021-09-23 - IMG 0034.jpg|[[SNCF Class Z 20900|Z 20900]] at Vauboyen  ([[RER C]])
File:Gare Bourron-Marlotte - Grez IMG 8615.jpg|[[SNCF Class Z 5600|Z 5600]] at Bourron-Marlotte - Grez ([[Transilien Line R]] and [[RER C]])
File:RER A - Gare AchèresVille 21.JPG|[[Altéo]] at [[Gare d'Achères-Ville|Achères-Ville]] ([[RER A]])
File:RER E - Gare Chelles 4Bis.JPG|[[Z 22500]] at [[Gare de Chelles-Gournay|Chelles-Gournay]] ([[RER E]])
</gallery>