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:

A woman dressed in early 20th-century fashion (while listening to music on her 21st-century smartphone) stands on the annual Shopper's Special subway line. Ben Gilbert/Business Insider

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!