The Travel Editor replies: A little-known walkway in Paris is La Petite Ceinture, or "the little belt", a stretch of railway line which forms a ring around Paris just inside the Peripherique (ring-road). There does not seem to be an organised body managing guided tours of the track and you cannot walk all of it. The popular area is in the 20th Arrondissement with access next to cafe La Fleche d'Or (102, rue Bagnolet. Metro: Alexandre Dumas). However, this track is something that the French tourist board in London refers to as "not strictly a tourist product". The line still belongs to the SNCF and isn't as "disused" as it appears. In parts, it is still infrequently used for freight transport, so is not entirely safe. La Petite Ceinture is more a curiosity than a walkway and it seems almost impossible to organise a tour through the SNCF, although it has been done for private groups.
More suitable is the Promenade Plantee situated above the Viaduct des Arts (tel: 00 33 166 75 80 66) at 9-129, ave Daumesnil. The nearest Metro stop is Bastille. The upper level of the viaduct has been converted into narrow gardens which are open to the public. This disused railway line also has a bicycle path. It runs 4.5km from the Opera Bastille to the Vincennes woods, passing embankments, railway tracks and small tunnels. The arches of the viaduct have been remodelled into shops, art galleries and workshops.