Grand Central Terminal Track 61, which FDR used to sneak in and out of Grand Central and hide
his disability (he had severe polio) from the public. Used other times by Presidents?

Matt Lauer of NBC put on his best play clothes May 8 2008 to examine “The Mystery of Track 61? on the Today show.
Lauer went 30 feet below the Waldorf to investigate the secret train track that has intrigued urban explorers for decades.

Lauer ended up with a nice 7-minute segment, with some commentary from colorful Metro-North spokesman Dan Brucker and Brooklynite historian Doris Kearns Goodwin. He spoke about not only the phantom track, but the mysterious bulletproof freight car still located under the Waldorf that played some sneaky role in presidential security.

"His armor-plated Pierce Arrow car would drive off the train, onto this
platform and into the elevator, and it would bring him and his car into the
hotel garage,'' Metro-North Railroad spokesman Dan Brucker said while
offering a rare tour of the long-unused station. "He could take the
presidential train back to Hyde Park without ever leaving the building.''

This story gets taller and taller anytime someone tells it.

The Grand Central Palace, the railroad's heating and power facilities and Adam's Express Co. occupied the area between 47th and 50th Streets and between Park Avenue and Lexington Avenue . They were torn down. In  1931, the Waldorf-Astoria completed its 40-story hotel on the site. Except for a small basement, the Waldorf-Astoria is directly over the tracks and the two platforms for the powerhouse and Adams Express.

The track 61 platform was of course never used or intended to be used in regular passenger service, and it was not even built for the hotel; it just happens to be there.

A stairway and a freight elevator run from the platform to a street entrance on 49 St. The freight elevator is not original and was probably built after the power house was torn down.
There is also another stairway exit, without an elevator, on the 50 St side of the hotel building.
So it amounted to a private railway siding underneath the building. Guests with private rail cars could have them routed directly to the hotel and take a special elevator  directly to their suites or to the lobby.

The baggage car ( "bulletproof freight car") was left by Penn Central for worktrain service and the MNCX reporting mark was painted on the car in 1984 in
North White plains (not by the Secret Service).

Also on the level:

In 1965, the platform was used for one of Andy Warhol's underground parties. In 1946,
the American Locomotive Company's 6000-horsepower Diesel-electric
locomotive that was headed to Santa Fe RR system to be used between
Chicago and Los Angeles, was exhibited on the Waldorf's private siding
beneath the hotel.