La voie des Fêtes et la voie navette

The Voie des Fêtes and the voie navette of the Paris Métro are two single-track connecting tunnels between lines 3 bis at the Porte des Lilas station, and 7 bis at the Place des Fêtes and Pré-Saint-Gervais stations, in the 19th arrondissement of Paris. The Voie des Fêtes takes its name from the station where it originates and it is on this track that the Haxo ghost station is located. The Voie navette takes its name from a shuttle that ran between the termini of lines 3 and 7 (now 3 bis and 7 bis) from 1921 to 1939. During the 1950s, it served as a testing ground for the RATP, notably for the first tests of automatic piloting in 1951, and then for the world's first rubber-tired metro prototype, the MP 51, from 1952 to 1956.

Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).