Flowers, plants and bright artistic creations decorate the entrances to their vaulted workplaces. “We’ve carved ourselves into the Petite Ceinture,” Sylviane Borie said with a laugh. She is part of a loose collective of sculptors, rap musicians and painters who have “created links with the community and camaraderie amongst artists,” she continued, “but our voice isn’t being heard.” Like the Petite Ceinture, the collective is in legal no man’s land and is facing eviction.

Peeling back a sheet-metal partition that was blocking access to another construction site, we ascended a long ramp that led up to a fence separating the site from the tracks. We found an opening in the fence, and after hoisting ourselves over it, we were back on the tracks. We soon realized we were above the Paris that we had seen a thousand times from ground level. From our elevated perspective we could see the Parisians on the way to their Sunday market, waiting for the bus, and children playing.

We walked under an office building that straddled the tracks as if it had grown up around them. Like undergrowth that gradually consumes a fence, the city had nearly overrun the Petite Ceinture. As Paris slowly rose up around us, we approached a comparatively open area where people were clearly living. Looking up from our trench we spotted the western edge of the Parc des Buttes Chaumont, an old quarry with a 100-foot bluff.

Among the signs of inhabitation: A beat-up stroller, a tree strewn with thin strips of paper containing various “wishes” and an impromptu “Home Sweet Home” sign quickly confirmed our hunch as we were greeted by residents of this section of the Petite Ceinture. With a hello and a look at their makeshift home, which resembled a tool shed carved into the steep embankment, we continued on. But after a glance down the line, our hearts dropped. The unwelcoming entrance to a railway tunnel was littered with broken bottles and other discarded waste. We had come prepared with headlamps and solid hiking boots, which made things easier. Although there was not much water to contend with in the tunnel, the footing was slippery, the temperatures chilly and the air damp.

A speck of light grew, and after a half-hour we reached the end. But after poking our heads out briefly, we were dismayed to see another tunnel ahead. This one would take us under the Père Lachaise cemetery, the final resting spot for many of France’s biggest celebrities — and Jim Morrison. Together the tunnels stretch nearly one and a half miles, so after crossing much of the 19th and 20th Arrondissements underground and finally reaching sunlight again, we felt we had earned our first real break. We shed our packs and enjoyed a sip of water — after more than three hours, we were about halfway through the first quarter of the railway.