Subterranean city

In 2012, a camera-toting Kate Seabrook was riding the U7 - Berlin's longest subway line - en route to capture a hidden corner of the city when she changed tact. "Why don't I document the journey instead of the end," she thought.

Having lingered on some wildly assorted platform facades, Seabrook believed she had discovered a window into Berlin that few others had noticed.

The Australian native was so inspired that she soon snapped all 173 Berlin subway stations, taking a photo at each stop before catching the next ride five-or-so minutes later.

The result of this exhausting underground journey is a photomontage that reveals Berlin's subway network to be a richly detailed palimpsest of the city.

A sprawling, beautiful time machine

Seabrook and I are surveying the classical adornments of the U3 platform at Fehrbelliner Platz, a station opened in the west of the city in 1913.

We note the ornate ceramic tiling, paid for with the taxes of the then-thriving local bourgeoisie, before wandering up to the bizarre, Lego-like blue and red station entrance from 1971, backdropped by some large gray Nazi buildings.

"That's what I find really interesting. It's kind of like a time machine," Seabrook says. "Here it's early 20th century and then you go downstairs and you're in the 70s. I don't think you really get that at any other change over station in Berlin."

The lower U7 platform at Fehrbelliner Platz is garishly orange and yellow, with futuro-kitsch designer touches including huge arrows showing the train's trajectory.

Built in the GDR years as a free-world alternative to the East German-controlled Ring line that runs parallel, the U7 typically reflects Berlin's historical and architectural disjunctions.

"You see it going from 70s pop art to 80s postmodernism," Seabrook explains. "It's like a timeline of design in a way."

One of her favourite subway stops is Pankstrasse, which doubled as a nuclear fallout shelter when opened in 1977 at the height of Cold War apocalyptic fervor - as did Hermanstrasse at the southern end of the same U8 line.

A few thousand people could survive in the station for weeks. Seabrook and her boyfriend joke that, if things get tough in Berlin, they can always move in.

Signposts in a subterranean city

I remember boarding the U8 subway for the first time on my way into Berlin from the airport and peering at the motley, relaxed crowd, some drinking beer, none wearing suits. At that moment I felt that I had arrived, that I could really see Berlin.

There is something honest, present and revealing about underground train travel - in any city. But in Berlin, the blue U sign is a particular marker of each distinct Kiez or neighbourhood.

A relatively young metropolis, Berlin's tenement-scape and train network grew up together, the city mushrooming organically around a star-shaped subway labyrinth. In Berlin pleasures of eating, drinking, shopping or dancing are all entwined with the ubiquitous U.

Though the London and Paris metros might have more stations, Berlin has a bigger system per head of population, and is the only "overpotent" transport network in Europe. This Untergrund was built for a city nearly twice the size - until it was all but destroyed in 1945.

And then came partition. The Wall was good for the subway, however, as it derailed earlier plans to turn Berlin into a car city. Without the 'anti-fascist' barrier, much of Kreuzberg, for instance, would now be a freeway.

As West Berlin was adding design flair to its new internal underground lines, the East Berlin authorities took up the challenge of bold subway decor in the divided years.

The 750th anniversary of the city was celebrated by placing historical maps of Berlin (at least the eastern half) on the walls of Märkisches Museum station in 1987-88, only for the politicized maps to become obsolete when the Wall fell barely a year later.

The GDR regime did not, however, decorate East Berlin's infamous "ghost" stations that lay on subway lines cutting from West to East to West again. These were tombs, frozen in time, lifeless apart from bored armed guards.

But they were also yet another subterranean Berlin signpost, a potent symbol of division, and of unification when they glinted back to life in the early 1990s.

Memorial to a misshapen metropolis

A single picture of a single Berlin station tells world-altering stories, as Seabrook began to realize.

Still admiring the classical lines of the U3 platform at Fehrbelliner Platz, Kate points out the framed black and white photos of old Berlin street scenes that line the finely tiled walls.

They are the work of Heinrich Zille, the legendary documenter of Berlin's slum-like tenement districts, and were installed in 1986, also as part of Berlin's 750th anniversary.

As many of these poor districts lay in the East, maybe this was a gesture of reconciliation - or maybe it was simply an opportunity to advertise the higher benevolence of the democratic free West?

"It's really interesting that his photos of the proletariat have been put in this prestigious station in this rich kind of district," Seabrook says. "It's another example of Berlin coming to terms with its past."

Kate has obviously discovered much about this city since looking longer at its subway. "What better way, not only for me to get to know Berlin, but for other people," she says. "I've had a lot of people say to me that they've never really looked at it before."

For under three euros, the Berlin underground offers not only a trip from A to B, but a journey in time, a chance to discover, to prize the facade of this misshapen metropolis.