(CNN)An unmarked, nondescript door off Park Avenue in Midtown Manhattan leads down to a shadowy train tunnel with a storied past.
Secrets of New York: Track 61 and Grand Central's M42
It once covertly carried a United States president from Grand Central Terminal to the Waldorf Astoria hotel.
Follow the tunnel today, and you'll find it still leads to the railroad car used by POTUS number 32, Franklin D. Roosevelt.
This is Track 61, the first stop on a secret subterranean tour of New York enjoyed by a very lucky few, as tourists and locals hurry about on the streets above, oblivious to those standing in the midst of history below.
The dusty rail car parked in the tunnel transported President Roosevelt's 1932 Pierce-Arrow limousine during his 16 years in office, says tour guide Danny Brucker in a distinctly New York accent.
The president would have been seated inside the limo as the train arrived, he says.
Brucker -- Metro-North Railroad's tour master and storyteller extraordinaire who ends most sentences with a "right, check" -- explains that this was one of the many ways a polio-stricken FDR masked his disability.
With Secret Service on the Pierce-Arrow running boards, the president would be driven from the train onto a freight elevator capable of carrying 8,000 pounds.
Brucker points out the rail car's bulletproof gun turrets and extra-wide, limo-sized pocket doors.
It sits just a few hundred feet from the enormous elevator, which still goes directly up to the hotel, which opened in its current location in 1931.
Even today, the site is largely blocked off from public access.
Brucker describes those who normally get the tour, as "super- and ultra-VIPs -- congresspeople, senators, celebrities."
"U.N. dignitaries are coming later in the week," he says.
Can he name names? Nope.
"And hurry up! Right, check;" there's much to see on this rare tour.
Although the Waldorf Astoria occupies the property above the track, the track is owned by Metro-North.
You kind of have to know someone, or be persuasive and resourceful, to secure your own look at this largely forgotten space.
Next stop: Grand Central Terminal.
Here, Brucker leads the group through the Main Concourse, crowded with some 750,000 commuters each day, to another unmarked door and another descent into the unknown.
This time past millions-year-old bedrock bearing dynamite blast markings, down nine flights -- the equivalent of 13 stories below street level -- to a secret sub-basement that "doesn't exist on any plans."
Adolf Hitler plotted to destroy this secret space, known as M42.
A vast monument to the Machine Age, M42 features mammoth electrical rotary converters that kept Grand Central humming during World War II.
Since Hitler knew about the sub-basement, armed guards were instructed to "shoot on sight," according to Brucker, "anyone entering carrying a bucket of sand. "
Sand thrown into the superheated converters "would've turned into a concrete block of glass" and shut down the trains that moved "80 percent of the troops and war materials," says Brucker.
The FBI foiled two German saboteurs sent by U-boat before they could execute an attack, according to Brucker.
M42 also houses an electric device built by Westinghouse in 1913 that spit out ticker tape noting a stalled train's exact location.
And even though radio communication made the engineering marvel obsolete in 1922, every so often "Apple [computer] chiefs quietly" come to "pay homage," says Brucker.
It's back up the stairs and into an elevator.
"I don't know if we can see this, but hurry," says Brucker, navigating another labyrinth of stairs and hallways.
He disappears into a hallway, reappears and motions for the group to follow.
"No phones. No cameras. And quiet!"
Suddenly we are in Metro-North Operations Control Center, an enormous room that features a swooping wooden ceiling.
Two giant screens take up one wall displaying the 700-some trains coming in and out of the terminal, different colors representing different lines.
The sheer volume is dazzling, as are the rows of women and men keeping the whole shebang humming.
"Onto the final stop," Brucker barks.
More maze-like walking to a door with a small window.
Like the other secret doors on this tour, it doesn't look like much.
But once opened? You're walking on a glass-floored catwalk at the very top of three enormous arched windows flanking Grand Central.
Brucker advises the group to kneel and look out.
The 1913 architectural triumph spreads out below, ant-like people moving across the elegant pink marble floor.
Being so close to Grand Central's majestic ceiling mural depicting constellations of the night sky is magical.
As the tour finishes, it's clear New York City has its own largely invisible stars, Brucker bright among them.
So, how do you get on this incredible tour?
"Email me," says Brucker. "Bug me."
Getting in touch with him? Well, that's part of the secret, but not impossible to figure out.