As cities grow and change, complex networks of elevated concrete highways and railways sprout up like vines, twist around each other and radically transform the space beneath them. Formerly vibrant urban districts are shrouded in darkness, and the potential to use that space is often wasted as officials fence it off or incorporate hostile features into the infrastructure to ward off loiterers and people lacking housing. Over time, some of those elevated roads might become obsolete, making the whole area feel like an urban wasteland.

But the need to make use of every available square foot of land is intensifying – and city planners working on the viaducts and overpasses of the future should probably take note of how that land is currently being reclaimed and rehabilitated to enhance its value to surrounding communities.

After the success of the High Line in New York City, an elevated linear park running along a former New York Central Railroad spur, many cities have begun transforming their own underpasses, viaducts, abandoned highway sections and even the tops of tunnels into verdant public spaces.

Atlanta’s BeltLine, Detroit’s Dequindre Cut and Washington D.C.’s planned 11th Street Bridge Park all demonstrate the how valuable the land can be to residents living nearby once it’s reactivated. These underpass parks can be surprisingly vibrant, like the 8-acre Underground at Ink Block park in Boston, Houston’s Sabine Promenade (top) or Toronto’s Bentway, which includes a 720-foot ice skate trail. Skate parks, like Portland’s Burnside ramps, are a natural fit.

Art installations brighten up cavernous underpass spaces, whether with colorful lights like San Antonio’s temporary Ballroom Luminoso installation by JB Public Art or with oversized sculptural elements like the flowers of Glasgow’s Phoenix Park. Some underpass spaces draw regular crowds as venues for movies or events, like Folly for a Flyover by Assemble in Hackney Wick, England.

In Yokohama, Japan, a notorious red light district flourished beneath an overpass for decades before authorities wiped out it, turning a bustling (if crime-ridden) area into a ghost town virtually overnight. A recent redevelopment project called the Koganecho Centre tucks a complex of new buildings into this underutilized space to make it functional for residents in a new way, adding an art gallery, a cafe, a meeting space, an artist’s atelier and an open-air piazza to a 328-foot stretch under the concrete arches.

Housing can take shape around and beneath viaducts, too. In 2005, Zaha Hadid completed the Spittelau Viaducts Housing Project as part of a waterside revitalization scheme in Vienna, Austria. A three-part structure of apartments, offices and artist studios winds through, around and beneath a disused railway viaduct, playfully interacting with it while creating a contrast between old and new. Even tiny slivers of land beside viaducts can avoid feeling dwarfed, darkened and constrained by the infrastructure when cleverly designed, like the narrow Archway Studios live-work space by Undercurrent Architects or the Koops Mill mixed-use development occupying a former brownfield (both in London.)

Unfortunately, even when they become magnets for pedestrians, cyclists, families and tourists, these urban revitalization projects aren’t all sunshine and rainbow bike racks. Some of them perpetuate cycles of displacement, pushing low-income and other marginalized populations further away from amenities instead of serving them. Urban infrastructure projects are often built in poorer areas of town in the first place.

Transforming empty space into parks and venues might improve them, but it might attract deeper-pocketed buyers to the area, too. The High Line, for example, is currently struggling to make up for the imbalances it has created in once-affordable areas of Manhattan’s Chelsea neighborhood. Incentivizing affordable housing developments along with all the other elements of an underpass or viaduct makeover could help build equity into these projects from the beginning phases.

Before the abandoned railroad tracks of Paris officially disappear – which is bound to happen, as cities realize the potential for reclamation as High Line-style parks – some intrepid locals are taking the opportunity to explore. “Train Project” by French design studio HeHe is a series of fun, quirky handmade vehicles that traverse the city on a line that most local residents never get to experience firsthand.

The autonomous vehicles include ‘Metronome’ and ‘M-Blem,’ two transparent, cylindrical solar-powered electric vehicles, and the sailboat-inspired ‘Radeau de Sauvetage.’ HeHe also developed additional vehicles made for train tracks in other cities, like an individual transport system to transfer objects from one place to another in Slovakia, and a one-person platform making use of the urban tracks in Istanbul.

HeHe calls The Train Project “an ongoing body of speculative investigation into the language and aesthetics of transport culture,” proposing personal rail travel as “a temporary, imaginative solution to question our industrialized conscience in relation to locomotion.”

The collective invited members of the public to travel on the vehicles, “journeying through space and time to experience the past and future of local transportation networks.” The idea is to take a picturesque trip through areas of the city that can’t be seen from the roads, reclaiming spaces that have been left to deteriorate until they’re inevitably integrated back into the functional urban landscape.

A similar project (above) explored sections of the 5,500-plus miles of abandoned railroad tracks in Mexico with the retro-futuristic SEFT-1, modeled after a midcentury vision of a spaceship.

Among the rusting vestiges of Europe’s Industrial Revolution, this 20-mile relic of a railroad in Paris has been abandoned since 1934, and while it has been retaken by nature in the ensuing decades, it will likely disappear altogether as the city grows. Photographer Pierre Folk has spent the last few years capturing what remains of ‘La Petite Ceinture,’ known as ‘Little Belt’ in English.

Completed in the 1860s, the railroad carried passengers around the City of Light until the underground metro system and personal automobile made it obsolete. At its peak, it was considered a hallmark of progress, with transfers to other Paris railway networks completed by horse-drawn carts. But once it was decommissioned, access was restricted, turning it into a sort of post-apocalyptic time capsule.

In the eighty years since it was last in use, La Petite Ceinture has become a habitat for at least 70 different animal species and host to over 200 varieties of plants. Folk’s photographs reveal a strange sort of in-between phase in which this stretch of railway in such a vibrant, metropolitan city is nearly forgotten, tangled with ivy.

As popular elevated parks like the High Line in New York City and Paris’ own Promenade Plantee prove, it likely won’t stay this way for long. Proposals have already been put forth to transform the rail from its current eerily quiet state into public space. Says Folk, “As a river, its shores constantly change over time, but it persists… an intimate place where past and modernity make their acquaintance.”