Chemin de fer de Petite Ceinture

Paris's former Chemin de fer de Petite Ceinture, also colloquially known as La Petite Ceinture, was a circular railway built as a means to supply the city's fortification walls, and as a means of transporting merchandise and passengers between Paris' major rail-company stations. Beginning as two distinct 'Ceinture Syndicate' freight and 'Paris-Auteuil' passenger lines from 1851, these lines formed an arc that surrounded the northern two thirds of Paris, an arc that would become a full circle of rail around the capital when its third Ceinture Rive Gauche section was built in 1867.

Although the Syndicate-owned portion of the line was freight-only in its first years, after the creation of a passenger service from 1862, the Chemin de fer de Ceinture became Paris's first metro-like urban transport, and even more so after the 'Ceinture Rive Gauche' passenger-and-freight section began.

Source: Wikipedia