The Petite Ceinture de Paris (Small Belt of Paris) is a 32 kilometer double-tracked railway that was built from 1852 to 1869, encircling Paris just inside the current boundaries of the city. Its original purpose was to transfer freight between the various railroad lines coming into Paris from different directions, but soon it was also used for passenger trains.
Passenger service on the Petite Ceinture was discontinued in 1934, but the line was still used to transfer freight until the 1990s. Since then it has been more or less abandoned, but some of the tracks still exist and can be seen in various places such as the Parc des Buttes-Chaumont in the 19th arrondissement (first and second photos here in this post).
In other places, the tracks have been removed but the right of way has been preserved for use as a walking and jogging trail. This is the case in Auteuil, for example, in the southwest corner of Paris.
Parts of the old right of way can also be used for walking or jogging in the 13th arrondissement near the Chambord Tower, but other parts have been covered up by the park Jardin du Moulin de la Pointe, which was built on a platform over the old railway.
Since the Petite Ceinture runs roughly parallel to the new tramway T-3a and T-3b, there was a long controversy in the 1990s about whether or not the tramway should be built at all. Some people argued that it would be simpler and less expensive to reactivate the Petite Ceinture instead. The automotive lobbies also liked this idea because it would have caused less disruption of motor traffic.
Reduction of motor traffic was one of the objectives of the tramway project, however, so the tramway eventually won out. Also, the tramway stations are closer together and are right where people need them, unlike the old Petite Ceinture stations.
My photos in this post are from 2007, 2012, 2013 and 2015. I revised the text in 2021.