RER B is one of the five lines in the Réseau Express Régional, a hybrid commuter rail and rapid transit system serving Paris, France and its Île-de-France suburbs. The 80-kilometre (50 mi) RER B line crosses the region from north to south, with all trains serving a group of stations in central Paris, before branching out towards the ends of the line.

Châtelet–Les Halles station is a major train hub in Paris and one of the largest underground stations 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 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.