A forgotten public transport system in Paris, known as the Petite Ceinture, once thrived as a major means of connecting the city's train stations. Built between 1852 and 1869, it reached its peak during the 1900 World's Fair, when it transported 38 million travelers. However, by 1927, passenger numbers had plummeted to 7 million, and the line was largely abandoned by the end of the 1930s. The decline was driven by obsolescence: outdated equipment, infrequent service, and inconvenient station locations made it less competitive compared to newer forms of transport like the metro and buses. The Paris metro, opened in 1900, offered direct access to neighborhoods, while the Petite Ceinture, which ran around the city's perimeter, often had stations far from populated areas. Passenger service on the Petite Ceinture was suspended in July 1934, except for the Auteuil line. That same day, a bus line named PC was introduced to replace the abandoned rail routes. Freight traffic, however, continued for decades, with the line serving as a vital link for cargo and technical trains. Freight operations lasted until 1989, and even after that, some limited activities, such as the movement of train cars and tourist excursions, continued until the early 2000s. Despite its decline, the Petite Ceinture's physical infrastructure remained largely intact, with rails, tunnels, viaducts, and station buildings preserved for over a century. A notable exception is the Auteuil viaduct, which was damaged during World War II and later replaced by the Garigliano bridge, breaking the continuity of the route. Today, the Petite Ceinture has become an unexpected ecological haven in the heart of Paris. Over 220 species of plants and animals, including 254 plant species, 34 birds, and 231 insects, have been documented along the route. A university study in collaboration with the National Museum of Natural History has identified the line as a significant ecological corridor. However, not all parts of the route are accessible to the public, and some sections remain fenced off or under development. Since 2013, efforts have been made to transform parts of the line into ecological spaces and pedestrian paths, connecting parks and creating natural habitats. In the 15th arrondissement, a newly opened segment has created nearly 2 kilometers of walking access and 5 hectares of green space. The Petite Ceinture is neither a dead relic nor a fully restored green path—it exists in a hybrid state. Rusted rails sit alongside shelters for wildlife, and some sections remain closed due to disputes between residents, environmental groups, and land managers. Yet, this forgotten railway continues to surprise, as seen in the presence of rare species like the forest warbler, which nests near modern Haussmannian buildings. The Petite Ceinture stands as a testament to the slow, multi-faceted decline of an old infrastructure and the unexpected life that has taken root in its abandoned corridors.