On any given day 750,000 people pass through Grand Central Terminal, according to Metro North, making it one of the busiest hubs for commuters in the country.

Amid all that hustle and bustle, as throngs of commuters, employees and tourists pass through its cavernous Grand Concourse and shopping and dining areas, how many stop to observe their surroundings?

An eight-part series on “Secrets of America’s Favorite Places,” attempts to delve into the history of famous landmarks around the country to unearth little known facts, with each part focusing on one iconic structure.

While some of Grand Central’s secrets are known only to employees, others are open secrets. “Some are more accessible than others and some you can only unearth by talking to an expert or by finding an old out-of-print book,” explained Paul Sauer, senior series producer at Indigo Films for the Discovery Family Channel.

The most well-known unofficial guide to Grand Central is Daniel Brucker, a longtime Metro-North Railroad employee, who has been working at Grand Central for more than 25 years. Beginning his tour at the information booth in the Grand Concourse, Brucker described perhaps the most intriguing aspect and a well-known secret of Grand Central.

“Within the Terminal is a secret train station that was built during the Great Depression, (it) cost a lot of money and was built for one person only,” explained Brucker. President Franklin D. Roosevelt who had been paralyzed from the waist down by polio, a fact largely unknown to the public at that time, would arrive on his own private train to Grand Central.

Inside of train cars would be his limousine. On the secret platform, an elevator was custom-made to accommodate Roosevelt’s limousine and Secret Service bodyguards. The elevator would carry Roosevelt, his car and his entourage directly to the Waldorf Astoria hotel, unseen by the public, the press and the Waldorf Astoria employees. The now-abandoned train car still lies in the depths of the terminal.

A secret hiding in plain sight is inside the information booth in the center of the Grand Concourse. A bronze cylindrical structure inside the booth conceals an old spiral stairway that employees inside the booth use to get to the lower level of the terminal.

While looking at the giant clock on top of the booth one wouldn’t even realize it’s made of opal, and valued today at a few million dollars by auction houses. Acorn and oak leaf motifs adorn everything from detailing on walls to adornments on lighting fixtures because they were the symbol of the Vanderbilt family who built the original depot in 1871. One of the most visible rests atop the opal clock.

Are there more undiscovered secrets that are waiting to be found? “The place is so vast and we only had five days to film this. Imagine if someone spent six months, how much more would they find,” said Sauer.

The episode on Grand Central airs on the Discovery Family Channel at 6 p.m. Sunday.