Testfeld Reallabor Radbahn by fabulism+nuko PartGmbB

Under the viaduct: a new public space in Berlin

In 2014, the association Paper Planes e.V. envisioned the Radbahn project as a transformation of the space under the Berlin’s oldest elevated subway line—the 9 km-long U1 viaduct—into a corridor for slow mobility. In 2022, the landscape architecture firm fabulism+nuko won the tender to design the first 200 metres, now known as Testfeld Reallabor Radbahn. Located in the vibrant Kreuzberg neighbourhood along Skalitzer Strasse, the former car park has been reimagined as a hybrid public space—an innovative, covered linear park where slow mobility and urban nature meet.

Structure: the islands and the green buffer

The removal of car parks marked a key first step in transforming the space into a pedestrian- and cyclist-friendly zone, laying the groundwork for a new spatial concept: three distinct “islands” connected by a lush green buffer. This vision emerged from a multi-stage participatory process involving local citizens and residents. Through online submissions and a collaborative workshop, citizens shaped the idea of connected green spaces rather than isolated park niches.

The Interaktionsinel was designed as a vibrant social space, hosting activities such as sitting, playing, art installations, and public events—creating a hub for community and engagement. The Radinsel focused on functionality, equipped with dedicated infrastructure tailored to cyclists’ needs. A third island, became an Artinsel, curated by Paper Planes e.V., offers temporary installations to explain the project’s process.

Linking these zones is a lush green buffer—two linear sides planted with a diverse range of flora, enhancing the user experience by introducing elements of nature into the urban fabric. On one hand it establishes an ecological refuge for birds and insects that find shelter within this biodiversity corridor and on the other the buffer creates a protection towards the street. Over time, the viaduct is being transformed into a piece of ecological infrastructure, combining urban mobility with nature conservation.

A response to current and future climate challenges

In the face of the climate crisis, highly urbanised environments present new challenges—which this project addresses through a future-oriented design approach. Guided by this vision, it prioritises the minimisation of new building materials and demolition, mitigates the urban heat island effect, and promotes the reuse of rainwater.
Time plays a central role in the project—not merely as a factor in immediate implementation, but as a dimension of long-term transformation. As a 1:1 mock-up, the project serves as a dynamic platform for testing, adaptation, and renewal. It exemplifies how true sustainability lies in the capacity to evolve in harmony with the environment. Among the project’s strategies are:

Unsealing: A key intervention involved unsealing more than 250 m² of paved surface. The niches between the viaduct piers, once used as car parking spaces, were transformed into permeable green areas. In zones where paving remains, surfaces with open joints were used to encourage rainwater infiltration.

Reuse of materials: All surfaces were constructed using reclaimed materials. On the Interaktionsinsel, red concrete pavers from the former car parks were carefully removed and re-laid with open. On the Radinsel, existing concrete blocks were combined with new red bricks, resulting in a vibrant surface. Historic granite kerbstones were cut into blocks and repurposed as stepping stones, creating a special vegetation path.

Water management: The planting areas are designed to retain and gradually release rainwater, ensuring that shrubs and perennials can thrive even during dry periods—without the need for artificial irrigation. The terrain was modelled in such a way that the rainwater is channelled from the sides into the more densely vegetated and lower-lying areas. Additionally, a filter bed, created in collaboration with TU Berlin, has been installed to purify runoff from the viaduct, using a combination of vegetation and layered substrates.

Hardy plants: A total of 90 shrubs and over 4,000 perennials and grasses were planted, many of them Mediterranean and climate-resilient, resulting in a robust, dynamic landscape capable of withstanding droughts conditions. The soil was created using a mix of techniques and substrates, forming a mosaic of 8 distinct zones. These micro-environments vary in exposure, substrate depth, moisture levels, and permeability, promoting a rich diversity of plant life. This ecological variety also provides habitat for animal species that would otherwise struggle to survive in the urban context.

Smart infrastructure: Interactive objects such as seating, sound installations and information systems are intended to make the public space integrative and dynamic.

Testfeld Reallabor Radbahn is not just an urban regeneration project, but a vision on a human scale that combines mobility, ecology and sociality in an innovative space.

• All landscape architecture offices involved in the design of landscape:
fabulism+nuko PartGmbB
(joint venture of fabulism GbR and nuko PartGmbB)

• Other credits:
Botanic consultancy: Christian Ranck
Client: Reallabor Radbahn gUG (haftungsbeschränkt) i.L.