Réseau Express Régional in the context of "Paris Métro"

Paris Métro Line 1 (French: Ligne 1 du métro de Paris) is one of the sixteen lines of the Paris Métro. It connects La Défense in the northwest and Château de Vincennes in the southeast. With a length of 16.5 km (10.3 mi), it constitutes an important east–west transportation route within the City of Paris. Excluding Réseau Express Régional (RER) commuter lines, it is the busiest line on the network with 181.2 million travellers in 2017 or 496,000 people per day on average.

The line was the network's first to open, with its inaugural section entering service in 1900. It is also the network's first line to be converted from manually driven operation to fully automated operation. Conversion, which commenced in 2007 and was completed in 2011, included new rolling stock (MP 05) and laying of platform edge doors in all stations. The first eight MP 05 trains (501 through 508) went into passenger service on 3 November 2011, allowing the accelerated transfer of the existing MP 89 CC stock to Line 4;. The conversion allowed Line 1 to operate as the system's second fully automated line, after Line 14.

Line 4 (French pronunciation: [liɲᵊ katʁᵊ]) is one of the sixteen currently opened lines of the Paris Métro, as well as one of its three fully automated lines. It connects Porte de Clignancourt in the north of Paris, to Bagneux-Lucie Aubrac in the eponymous southern suburban city, on a north-south axis across the French Capital. Held within the boundaries of the City of Paris for over a century – until its southern terminus was changed from Porte d'Orléans to Mairie de Montrouge in 2013 – the line was sometimes referred to as the Clignancourt – Orléans Line. At 13.9 km (8.6 mi) in length, it connects with all Paris Métro lines apart from the very short 3bis and 7bis branch lines, as well as with all 5 RER train lines. It also serves three of the Paris Railway stations, Gare du Nord, Gare de l'Est, and Gare Montparnasse. It is the second-busiest Métro line after Line 1, carrying over 154 million passengers in 2004.

Line 4 was the first line to connect to the south side of the River Seine, through an underwater tunnel built between 1905 and 1907. Line 4 long ran the longest-serving MP 59 rubber trains, which used rheostats to dissipate the braking power through resistance, making line 4 the hottest line in the system. Those trains were withdrawn from service on Line 4 during the course of 2011 and 2012 after 45 years (with some being in service for 50 years), yet remained another 12 years on in service on Line 11. On Line 4, they were replaced by the MP 89 CC stock transferred from Line 1, right after its automation.

Châtelet–Les Halles station (French: [ʃɑtlɛ le al]) is a major train hub in Paris and the largest underground station in the world. Opened in 1977, it is the central transit hub for the Paris metropolitan area, connecting three of five RER commuter-rail lines and five of sixteen Métro lines. The hub hosts 750,000 travellers per weekday (493,000 for the RER alone) and platforms separated by up to 800 metres (0.5 mi). It is named after the nearby Place du Châtelet public square and Les Halles, the former wholesale food market of Paris, now a shopping mall.

Les Halles (French pronunciation: [le al] ; 'The Halls') was Paris's central fresh food market. It last operated on 12 January 1973 and was replaced by an underground shopping centre and a park. The unpopular modernist development was demolished yet again in 2010, and replaced by the Westfield Forum des Halles, a modern shopping mall built largely underground and topped by an undulating 2.5 hectare canopy. The mall sees around 50 million visitors every year, making it the busiest in France as of 2019. It is directly connected to the massive RER and métro transit hub of Châtelet–Les Halles, Paris's busiest station.

The Elizabeth line is a railway line that runs across Greater London and nearby towns, operating similarly to the RER in Paris and the S-Bahn systems of German-speaking countries. It runs services on dedicated infrastructure in central London from the Great Western Main Line west of Paddington station to Abbey Wood and via Whitechapel to the Great Eastern Main Line near Stratford; along the Great Western Main Line to Reading and Heathrow Airport in the west; and along the Great Eastern Main Line to Shenfield in the east.

Under the project name of Crossrail, the system was approved in 2007, and construction began in 2009. Originally planned to open in 2018, the project was repeatedly delayed, including for several months as a result of the COVID-19 pandemic. The service is now named after Queen Elizabeth II, who officially opened the line on 17 May 2022 during her Platinum Jubilee year; passenger services started on 24 May 2022.

The Boulevard Saint-Michel (French pronunciation: [bulvaʁ sɛ̃ miʃɛl]) is one of the two major streets in the Latin Quarter of Paris, France, the other being the Boulevard Saint-Germain. It is a tree-lined boulevard which runs south from the Pont Saint-Michel on the Seine and Place Saint-Michel, crosses the Boulevard Saint-Germain and continues alongside the Sorbonne and the Jardin du Luxembourg, ending at the Place Camille Jullian just before the Port-Royal RER station and the Avenue de l'Observatoire. It was created by Baron Haussmann to run parallel to the Rue Saint-Jacques which marks the historical north-south axis of Paris. It is known colloquially as Boul'Mich' in French.

The boulevard serves as a boundary between the 5th and 6th arrondissements of Paris; odd-numbered buildings on the eastern side are in the 5th arrondissement and even numbers on the western side are in the 6th. It has a length of 1,380 m (4,530 ft), an average width of 30 m (98 ft) and takes its name from the Pont Saint-Michel.

S-Bahn (/ˈɛsbɑːn/ ESS-bahn, German: [ˈɛsˌbaːn] ; lit. 'S-train') is a type of hybrid commuter rail and rapid transit service, originating in German-speaking countries. In general, S-Bahn systems link suburbs with the city centre at moderate speeds, while continuing across the urban core over a central high-frequency corridor where multiple lines converge, where they provide a rapid means of travel across the city. The name S-Bahn derives from the German terms Schnellbahn (lit. 'rapid train'), Stadtschnellbahn (lit. 'rapid city train'), and Stadtbahn (lit. 'city train'; not to be confused with the modern Stadtbahn).

The first such network, the Berlin S-Bahn, began operation in December 1930 and inspired similar systems across Europe. The term "S-Bahn" has since become a generic term for hybrid suburban–urban rail networks, with counterparts such as the S‑tog in Denmark, S‑Trein/Train S in Belgium, Esko/S‑lines in the Czech Republic, Linee S in Italy, and SKM in Poland. The S-Bahn is also recognized as a train category in several European countries. The concept also influenced systems outside the German-speaking world, including the Réseau Express Régional (RER) in Paris and its counterparts in other cities.