Every one of the six stations on Düsseldorf’s Wehrhahn line provides an artistic experience. You can follow a poem into the underground, view the Amazon from above, glimpse into the universe, listen to alien sounds or be digitally beamed into an installation. Connecting east and west, the 3.4km underground line through the heart of the city transported its first passengers in February 2016, after nine years and €844m of construction work. It is not the world’s largest public transport project – you can get from one end to the other in ten minutes – but size was never the point. The Wehrhahn line is one of the most successful marriages of architecture and art – from the very first drawing, every station was developed together with an artist.

Netzwerkarchitekten, based in Darmstadt, won the public tender to develop the stations, and co-director Markus Schwieger calls the job a “dream project”. “We were a young bureau and participated in this competition in 2001 against larger firms,” he says. “The condition was that we would work with an artist. So first we worked with Heike Klussmann, and later the city council ran another competition to find five further artists, one for each station.”

It was the collaborator involved the longest in the project who developed the grand finale (or starting point, depending on your direction of travel). Heike Klussmann’s Surround at Pempelforter Straße features geometric shapes and stripes running against the grids of the station’s various surfaces, playing with lines leading from the four entrances throughout the station.

At Schadowstraße, Ursula Damm employed various elements for Turnstile, including using cameras to beam passers-by down to the platform as virtual-reality characters mixed with aerial views of Düsseldorf.

Giving the eyes a rest, Ralf Brög’s sound installation at Heinrich-Heine-Allee was developed with a broad group of experimental sound engineers.

At Graf-Adolf-Platz, Manuel Franke’s Achat uses green glass panels to cover the entire station. Where the green gives way in this all-encompassing work, visitors get a glimpse of a second layer in deepest purple, resembling a bird’s-eye view of the Amazon with its dark rivers flowing through the green jungle.

Back at the other end, at Kirchplatz, Enne Haehnle has scribbled poetry in 3D along the entrances and down to the tracks – she calls it Track X.

The penultimate station takes a further step away from Earth. Thomas Stricker’s conceptual spaceship at Benrather Straße, called Heaven Above, Heaven Below, turns this humble underground station into something with intergalactic reach, and it’s one of the most popular pieces in the project. “I didn’t want to build a cave,” says Thomas. “I wanted to reverse the imagination: you go underground, but I wanted to bring the expanse of space down there. I know claustrophobia, and I wanted to counter that fear.” The team at Netzwerkarchitekten tried to match each stop with the artist’s temperament, and Thomas says the depth of Benrather Straße station made it the perfect fit for his extra-terrestrial work, giving him the opportunity to misdirect his audience: “It opens at an angle below. There is an oblique wall – and a certain amount of art is in that oblique wall. You would never expect that.”

Manuel Franke’s greened station also invites travellers to linger and marvel at the walls. “The green paint on the glass panels is distributed in a way that it shows the surface below. The even surface is being broken up – just like with rivers, the interesting areas are the shores. The green becomes more delicate and takes on the characteristic of natural stone.” For Manuel, who lives and works in Düsseldorf, the station still makes him proud: “There is a certain bewilderment that public transport could be given such an important role. We are very spoilt with art here – the Kunstakademie [art academy] delivers constant fresh talent.” He says even his daughter is excited: “She takes the Wehrhahn line to school, and sometimes she and her friends take selfies in ‘Dad’s station’."

Düsseldorf has long punched above its weight as a small but creative town. Fashion and advertising are major players, and the art world has flourished, not least because local rival, Cologne, nips at its heels as the regional artistic powerhouse. But the Wehrhahn line has put Düsseldorf streets ahead. “It was worth it – people identify with it,” says Markus Schwieger. “We sometimes do tours for visitors from other cities who look at it as a best practice example. The city’s football team even have the pattern of the line printed on their tops.” Gregor Jansen, the director of Düsseldorf’s contemporary art museum, Kunsthalle, adds: “The quality of this project is something we have seen only in the Moscow stations built during the 1920s. The beauty of these buildings is fantastic. The station designed by Heike Klussmann has a kind of dynamic you notice every day, and visitors can’t help but feel it, too.”