(CNN) — Just one meter below Budapest's UNESCO-protected Andrássy Avenue, yellow train carriages rattle into the busy platform at Oktogon Station.
The crowds board and alight at the tiled station before a klaxon like noise sounds and the doors slam.
Although the steam-run London Underground opened first in 1863, Budapest claims the first electric underground railway on the continent.
Europe's second underground railway made its inaugural journey more than 100 years and is still used by thousands of locals and tourists every day.
"It's not a metro, it's an underground railway," says Ibolya a worker at a museum commemorating M1, the yellow line.
Hidden behind the subterranean ticket office at Deák Ferenc tér, the Millennium Underground Museum is a memorial to the underground railway's golden age, where vintage brown and yellow cars rest on the original rails, with a cast of waxwork figures in navy conductor uniforms.
M1 earned its "Millennium Underground" title due to its role in the 1896 celebrations marking the 1,000 year anniversary of the Hungarian conquest of the region.
Buildings such as the Millennium Monument in today's Heroes' Square sprung up around the city for the occasion.
Budapest is home to the oldest eectrified underground railway system on the European continent.
This innovative railway quickly had to meet the demands of a growing city. Using the most modern construction techniques like mixers, pumps, and excavators, it opened under its tight deadline after just 21 months.
"Line two also has an interesting history," Gabi, another museum employee, adds, "there's a secret bunker just hidden off the Kossuth tér metro."
Between the Kossuth tér and Deák Ferenc tér metro platforms, a secret tunnel diverted away from the main track leads into the F-4 Object, a 3,500 square meter bunker 39 meters underground.
Commissioned by then Prime Minister Mátyás Rákosi in preparation for a nuclear attack, the concealed nuclear bunker was built during the construction of the M2, the red line, in the 1950s.
With room to accommodate 2,200 people, it was built with a direct passage from the Hungarian Parliament.
The government planned to escape via subway to the Keleti train station once the attack was over and leave Budapest in an armored train. While it was never used, the bunker was kept on standby until the 1970s.
Its existence only became public knowledge in the 1990s. It remains empty and unused, only visited for maintenance checks, or on very infrequent tours.
Infamous ticket inspectors
Budapest's M1 has been in constant operation since 1896 and is still used by thousands daily.
Most residents view the city's four underground lines as simply a way of getting from A to B, and have little (if any) knowledge of such details.
"I use the metro every day and I haven't noticed anything unusual about it," says tour guide Attila.
"The most common thing I see is a lot of people who don't buy a ticket and then are surprised when they have to pay a fine. It's so obvious that the service is not free… yet people still argue with the controllers."
When you mention the metro to locals, ticket controllers often come up.
The 2003 cult film "Kontroll" immortalized the metro's hauntingly gritty stations, with inspectors operating on a fictionalized transport network.
Shot entirely on M3, the blue line, the surreal movie hints at an underground ecosystem where illegal raves occupy platforms and controllers sprint for bets along dusty rails in front of the "midnight express".
M3 opened in the 1970s and runs 16 kilometers under the Pest side of the river, making it the city's longest metro.
It lacks the historic charm of the M1, but you can still spot curiosities as you change lines.
Bilingual poetry is scrawled on the tiled walls behind the red plastic seats at the Deák Ferenc tér platform, inscribed in Hungarian and Portuguese by artist João Vieira.
The tiles on this platform at Deák Ferenc tér metro station are inscribed with poetry.
Meanwhile a curious red marble sculpture from the 1980s, named "Birth of a City," lies at the Kálvin tér underpass.
While many feel it resembles female genitalia, this abstract artwork is actually a tribute to the city's expansion.
One of its most interesting parts is located in the bottom right corner, where a cat makes a cameo.
Supposedly modeled on sculptor Gyula Illés' pet Maci, urban legend has is that stroking the kitty's tail will bring you good luck.
Budapest's final metro line, M4 or the green line, took 40 years to build.
When construction eventually began in 2004, the Hungarian architects charged with developing its stations were keen for them to represent the city's modern spirit.
That vision was brought to light with cavernous halls crossed with imposing concrete beams, bold accents of color and, in the case of the Bikás Park station, floods of light through undulating glass panes.
After battling years of corruption and dispute, the line opened in 2014 and became a bastion of contemporary Hungarian architecture.
It may have taken significantly longer than 21 months to get off the ground, but the M4 now serves as a modern time capsule, keeping Budapest on the move.
Jennifer Walker is an Anglo-Hungarian writer and former physicist living in Budapest. She tweets at @JDWalkerWriter.
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