The New York City subway system is one of the most fascinating curiosities in a city full of mysteries. Miles of underground track shrouded in darkness, littered with abandoned stations and secret passageways — it's a common object of desire for the urban explorers among us.

And, occasionally, New York City acknowledges the delightful mystery surrounding its 24-hour transportation system. The annual “Shopper's Special” train line is a perfect example of this:

The train line, consisting of eight vintage New York subway cars from several different eras, runs for a few weekends each year — from the Sunday after Thanksgiving to the end of the year, only on Sundays.

So what'd we do this past weekend? We got on the train and took a ride!

I got on at the Second Avenue stop in Manhattan — the train runs between the Second Avenue stop in Manhattan and the Queens Plaza stop in Queens.

As you can see, the train runs all day starting at 10am and concluding at about 5pm.

Even though we arrived at 12:30, there were already a bunch of people waiting — some were clearly tourists; others were clearly New Yorkers.

A tonne of people on the train were dressed in period-appropriate clothing. Of note, these are not paid actors.

The gentleman here in the white hat told me that he and his crew were going to a party afterward at Webster Hall, an event space/concert hall in Manhattan's East Village.

But you're here for the train, right? So was I! It arrived about 10 minutes ahead of its 1.03pm departure time — plenty of time for photos!

Just like any NYC subway, the Shopper's Special rolls into the station at high speed.

Since the Shopper's Special line runs during the holidays, it's festooned with Christmas wreaths on the back and front.

While the train was stopped, people dressed in anachronistic clothing posed for photos next to the antique train cars.

There are some amazing details on these old train cars.

OK, enough is enough: It's time to get on this train and take a ride through history!

Right off the bat, the level of detail is stunning. Old advertisements run through each car.

Many of the advertisements on the first train car were from the 1940s, such as this advertisement for war bonds.

The cars are near-perfectly restored, from the metal “straps” to hold onto, to the yellow-orange seats.

(Ben Gilbert/Business Insider (Ben Gilbert/Business Insider)

The lightbulbs have all been replaced, and the ceiling fans are all running (pushing air out of the vents along the top of the car).

This car was built in 1932 by the American Car and Foundry company, so it's assuredly gotten some love in the past 80 years.

In addition to restoring the lighting and ventilation systems, the MTA also restored the station ID placard. Remember how there weren't always screens everywhere?

A lot of the fun is in the details. I couldn't stop gawking at every old advert.

There's something inherently more classy about calling it the “City of New York” instead of just New York City, isn't there?

Nearly 100 years later, and the New York subway is still running ads for New Yorkers (and tourists!) to visit Coney Island.

Some of the ads are for events long passed, like this “I Am An American Citizenship Day” — an apparently free event in Central Park.

(Ben Gilbert/Business Insider (Ben Gilbert/Business Insider)

And yes, Citizenship Day is a real American holiday that you've probably never heard of (I certainly hadn't). It takes place on September 17 every year and serves to commemorate the signing of the US Constitution (on September 17, 1787). The holiday was originally called “I Am An American Day,” which was celebrated during the 1940s; it became “Citizenship Day” in the early 1950s. Probably not a bad idea considering America's history as a nation of immigrants.

To the next car! The Shopper's Special keeps the doors between cars open, so you can freely walk through its eight cars.

The next car was even older, from 1930, also built by American Car and Foundry.

This is not a bathroom — this is for subway operators, despite looking like some sort of nightmare prison.

The sliding doors were far less safe on these early trains. If you got caught in between, it felt as if two metal doors were closing on you!

Despite the subway car being from the 1930s, advertisements in this car started erring toward the 1960s.

This older car looked a bit worse for wear — the metal “straps” were extra worn, and the fans were worryingly close to riders' heads.

The seats have clearly been replaced, but they still retain the same charm of their original form.

And our friends dressed in vintage clothing made another appearance, classic photography gear in-hand:

(Ben Gilbert/Business Insider (Ben Gilbert/Business Insider)

Stuff like emergency brakes are notoriously low-tech

There were some adorably bizarre seats on this first car.

It's pretty delightful seeing modern fashion juxtaposed with these classic subway cars.

Next car!

The next car was far more modern, but that's not because it was built much more recently than the other cars.

The “straps” were much newer, as was the lighting and the seats. This looked the closest to the modern New York subway.

The subway map looked considerably different back when this train last ran.

These cars ran through the 1970s — some of the riders were discussing when they used to ride on these trains in New York.

For the final car, another throwback to the 1930s (though the decoration on the interior is from the 1940s).

The final car looked more like a train line than the modern subway system.

It was full of the same adorably designed seats.

And the placards on this one even lit up.

My final look into the train was perfectly representative of the bizarre mash-up of antique train cars with modern life: a woman, dressed in antique clothes, listening to music on her smartphone.

The Shopper's Special train line runs every Sunday from 10am to 4pm, starting at the Second Avenue stop in Manhattan and completing at the Queens Plaza stop in Queens. It runs along the F/M line, making a handful of stops on the way.

Rides cost the same $2.75 that all subway rides cost, and you can take the train as many times as you'd like. But hurry up and do it sooner rather than later, as this unique subway line runs only through 18 December.

And if you miss it, don't worry too much — the entire subway line is normally on display at the New York Transit Museum.