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Liam Blank

Paris · Réseau express régional

Paris RER

By Liam Blank · Updated

A trip from home east of Paris to work west of it once broke in the city. The suburban train stopped at a terminal; the passenger had to cross Paris by another route. The RER, Paris’s regional railway, joined lines through the center so one train could carry the longer journey. New Yorkers weighing Penn Station plans can use this case to judge the destinations an investment would make reachable without another train change.

A route to the
other side of Paris.

Boissy-Saint-Léger is southeast of Paris. La Défense, the business district, is west of the city. A rider between them needs a railway across the center. The map follows how separate lines were linked in 1977, and how Cergy and Poissy later joined that route. The central tunnel turned stations where journeys ended into stops on the way to another part of the region.1

The journey breaks in Paris.

In 1972, western trains from Saint-Germain-en-Laye ended at Auber. Eastern trains from Boissy ended at Nation. To get from one side to the other, a rider had to cross Paris between the two train journeys. 2

RER A in 1972: a regional map and an enlarged central Paris viewThe western regional line terminates at Auber; the eastern line terminates at Nation. Their central Paris gap is visible in an enlarged view. Geography uses official Île-de-France Mobilités route and station data; selected stations are labelled.RER A · 1972N ↑Saint-Germain–en-LayeBoissy–Saint-LégerLa DéfensePARISCENTRAL PARIS · ENLARGEDLine in serviceTwo lines approach Paris.The center is still missing.AuberNationNo through routebetween these terminals5 km · regional mapRER A in 1972: a regional map and an enlarged central Paris viewThe western regional line terminates at Auber; the eastern line terminates at Nation. Their central Paris gap is visible in an enlarged view. Geography uses official Île-de-France Mobilités route and station data; selected stations are labelled.RER A · 1972N ↑Saint-Germain–en-LayeBoissy–Saint-LégerLa DéfensePARISCENTRAL PARIS · ENLARGEDAuberNationNo through routebetween these terminalsIn service
Selected RER A routes and stations at each date, drawn on current geographic linework. Paris municipality shaded; major rivers provide geographic context. Route and station data: Île-de-France Mobilités; boundary: IGN / data.gouv.fr; hydrography: AESN / IAU, Institut Paris Region, Licence Ouverte 2.0. Map sources.

East and west become one train journey.

The Auber–Nation connection, inaugurated on 8 December 1977, let trains cross the center through Châtelet–Les Halles and Gare de Lyon. A new branch also reached Noisy-le-Grand–Mont d’Est. Riders could continue beyond their old Paris terminal. 2

RER A in 1977: a regional map and an enlarged central Paris viewThe new central connection links Auber, Châtelet–Les Halles, Gare de Lyon and Nation. The eastern branch reaches Noisy-le-Grand. Geography uses official Île-de-France Mobilités route and station data; selected stations are labelled.RER A · 1977N ↑Saint-Germain–en-LayeBoissy–Saint-LégerNoisy-le-GrandMont d’EstLa DéfensePARISCENTRAL PARIS · ENLARGEDLine in serviceNew connectionAuberChâtelet–Les HallesGare de LyonNation5 km · regional mapRER A in 1977: a regional map and an enlarged central Paris viewThe new central connection links Auber, Châtelet–Les Halles, Gare de Lyon and Nation. The eastern branch reaches Noisy-le-Grand. Geography uses official Île-de-France Mobilités route and station data; selected stations are labelled.RER A · 1977N ↑Saint-Germain–en-LayeBoissy–Saint-LégerNoisy-le-GrandMont d’EstLa DéfensePARISCENTRAL PARIS · ENLARGEDAuberChâtelet–Les HallesGare de LyonNationIn serviceNew link
Selected RER A routes and stations at each date, drawn on current geographic linework. Paris municipality shaded; major rivers provide geographic context. Route and station data: Île-de-France Mobilités; boundary: IGN / data.gouv.fr; hydrography: AESN / IAU, Institut Paris Region, Licence Ouverte 2.0. Map sources.

More suburbs gain the connected route.

Cergy joined in May 1988 and Poissy in May 1989. Their trains could continue through Nanterre to La Défense and across Paris. The eastern branch had reached Torcy in 1980. The city-crossing connection could now serve more of the region. 2

RER A in 1989: a regional map and an enlarged central Paris viewThe Cergy and Poissy branches join the cross-Paris route through Nanterre. The Cergy terminus shown is Saint-Christophe, and the eastern branch reaches Torcy. Geography uses official Île-de-France Mobilités route and station data; selected stations are labelled.RER A · 1989N ↑Saint-Germain–en-LayeBoissy–Saint-LégerTorcyCergy–Saint-ChristophePoissyLa DéfensePARISNanterre–PréfectureCENTRAL PARIS · ENLARGEDLine in serviceNew connectionsAuberChâtelet–Les HallesGare de LyonNation5 km · regional mapRER A in 1989: a regional map and an enlarged central Paris viewThe Cergy and Poissy branches join the cross-Paris route through Nanterre. The Cergy terminus shown is Saint-Christophe, and the eastern branch reaches Torcy. Geography uses official Île-de-France Mobilités route and station data; selected stations are labelled.RER A · 1989N ↑Saint-Germain–en-LayeBoissy–Saint-LégerTorcyCergy–Saint-ChristophePoissyLa DéfensePARISNanterre–PréfectureCENTRAL PARIS · ENLARGEDAuberChâtelet–Les HallesGare de LyonNationIn serviceNew links
Selected RER A routes and stations at each date, drawn on current geographic linework. Paris municipality shaded; major rivers provide geographic context. Route and station data: Île-de-France Mobilités; boundary: IGN / data.gouv.fr; hydrography: AESN / IAU, Institut Paris Region, Licence Ouverte 2.0. Map sources.

Keep the same train
across two power systems.

Keeping a Cergy or Poissy passenger on one train across Paris required equipment that worked on two railways. SNCF, France’s national railway group, supplies 25,000 volts alternating current on these branches; RATP, Paris’s public transport operator, supplies 1,500 volts direct current on its section. Both use overhead wire. Dual-voltage trains, designed to use either supply, enabled the interconnection.2

The pantograph—the roof-mounted arm that collects power from the wire—lowers and rises during the change between Houilles–Carrières-sur-Seine and Nanterre–Préfecture. The train briefly loses the overhead supply, then takes power from the other system. Passengers stay aboard across a boundary that the equipment is built to handle.3

A double-deck MI09 train at the underground Nanterre-Préfecture platform, with passengers boarding and a departure display for Marne-la-Vallée–Chessy.
Nanterre–Préfecture, west of Paris. The display names Marne-la-Vallée–Chessy, east of the city: the destination is beyond the central railway. A double-deck RER train stands at platform 1C. Photograph: Remontees, 8 May 2024, CC BY-SA 4.0.
One railway journey, two overhead power systemsOn the Cergy and Poissy branches, trains use 25 kilovolt alternating current under SNCF operation. Between Houilles and Nanterre-Préfecture they briefly lose high-voltage supply as pantographs lower and rise for the transition to RATP’s 1.5 kilovolt direct current. The illustration shows an eastbound journey and is not an engineering cross-section.OVERHEAD POWERSNCFRATP25 kV1.5 kVAlternating currentDirect currentPantograph lowered.Power from the wire briefly off.HouillesNanterre–Préfecture
Power change on a trip from Cergy or Poissy toward Paris. Electrical symbols and the collecting arm are schematic; distances and equipment geometry are not to scale.3

Passengers stayed aboard.
The driver changed.

On the north–south RER B, Gare du Nord joined two railways. A through train still needed a driver qualified for the next section to take over there. Passengers could stay aboard, but departure depended on the replacement being ready. Qualifying drivers for the whole line removed that handover from the stop.4

RER B at Gare du Nord
DateHow the trip workedWhat happens at the boundary
1981Change across the platformThe passenger changes trains.
1983One train across ParisOn a through service, the passenger stays aboard; the driver changes.
2009Same driver throughoutThe passenger and driver continue on the same train.

RER B’s stages of connection at Gare du Nord. Joint RATP–SNCF line committee, 29 November 2010, p. 4. Line A has its own operating arrangements.4

RATP and SNCF still run different sections of lines A and B, overseen by Île-de-France Mobilités, the regional transport authority. Delivering one passenger journey therefore requires a service arranged across two operators.5

Choose the trips
Penn should connect.

Ask the rail agencies to show a named cross-region journey a Penn plan would enable: where the rider starts, where the train continues beyond Midtown and how the timetable carries that person through. Compare those journeys before permanent track and platform choices are fixed. Equipment compatibility and driver arrangements belong in the same service plan. Penn’s capacity and reliability must be demonstrated on its own railway.

Explore Penn’s service choicesRead the Thameslink case
Sources & image credits
  1. RATP, “Des lignes d’histoires”. Inauguration of the Auber–Nation central link and extension of RER B from Luxembourg to Châtelet–Les Halles, 8 December 1977. RATP / SNCF, “Une longue histoire débutée en 1969”, 19 April 2024: replacement of the Bastille terminus by the Nation–Boissy line.
  2. RATP / SNCF, “Le RER A : une histoire entre RATP et SNCF”, 4 November 2016. Western and eastern openings, central connection, Cergy and Poissy interconnections, dual-voltage MI84 equipment and the later Torcy, Chessy and Cergy-le-Haut extensions.
  3. SNCF RER A, “Que faut-il pour faire circuler un train ?”, 23 January 2015, “La caténaire”; RER A, “Ventilation réfrigérée…”, 15 May 2026, official staff response by Ugo. Overhead supplies, the transition between Nanterre and Houilles, and the brief interruption of high-voltage supply as pantographs lower and rise.
  4. RATP / SNCF, RER B line committee, 29 November 2010, p. 4; pp. 14–15 on driver training. Passenger interchange in 1981, train interconnection in June 1983 and end of the driver handover in 2009.
  5. RATP Group, “Quelles lignes sont coexploitées par la RATP et la SNCF en Île-de-France ?”, 10 July 2025. Current allocation of the A and B lines between the two operators.
  6. Île-de-France Mobilités, geographic railway traces and station locations, accessed 9 October 2026, Licence Ouverte 2.0. Paris boundary: data.gouv.fr geographic API, derived from IGN administrative boundaries. Major rivers: AESN / IAU, Institut Paris Region, simplified hydrographic network (1999 geography), Licence Ouverte 2.0. Historical route selection follows the operator histories above; current geometry does not reconstruct historical track alignments. Photograph: Remontees / Wikimedia Commons, CC BY-SA 4.0.