The Secret Train Platform Beneath The Waldorf-Astoria
the long-abandoned Waldorf-Astoria train platform, which allowed VIPs to enter the hotel in a more private manner—most famously it was used by Franklin D. Roosevelt, possibly to hide the fact that he was in a wheelchair suffering from polio. The mysterious track, known as Track 61, still houses the train car and private elevator, which were both large enough for FDR's armor-plated Pierce Arrow car. Legend has it that the car would drive off the train, onto the platform and straight into the elevator, which would lead to the hotel's garage.
Some fun facts regarding the timeline of the tracks: It was first used by General Pershing in 1938, and less than 30 years after that, in 1965, it was the venue for a party thrown by Andy Warhol (fittingly called The Underground Party).
This space will likely never be open to the public... unless you're a squatter—"by 1978, the platform was known as one of the many places in Grand Central Terminal where squatters lived." However, current construction on the new LIRR extension has probably taken care of that (though despite the construction, MTA spokesperson Dan Brucker tells us the train car will remain).