The last digit is a "2." The History Channel's new series "Cities of the Underworld" covered this train car in its NYC episode, and also alluded that it may have been FDR's train car. The guide in the episode said that the car had turned up a few months after FDR's death, and that the MNCX designation meant that the car was privately owned, usually by a government party.
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This car has nothing to do with the so-called "FDR" car. This flat car is what is known as an "idler," and is used in conjunction with a railroad crane. When not in use the boom hangs over the idler car.

The "FDR" car was a business car, which is a custom-fitted. luxury coach. It was complete with bedrooms, dinning room, fully equipped kitchen and a lounge area at the rear of the car.

Me again. I just researched railroad designations online. MNCX="Minnesota Corn Processors"? Okay, why is that registry slapped on the side of a car with foot-thick walls and windows under the Waldorf-Astoria? Curious....

Interesting; on 8/9/06, WYNY broadcast a story about an armoured rail car from the 1940s, found abandoned on Metro North's Track 61 under the Waldorf-Astoria Hotel north of Grand Central Station. Speculation is that it's the train car that FDR's automobile traveled in when he came to New York.

The last digit of her registry is obscured by a station logo in the TV report, but starts MNCX-00(?); the reporter states that that final "X" denotes a military origin. Might this flatcar also be a piece of presidential history? :)

http://www.myfoxny.com/myfox/pages/Home/Detail?contentId=450940&version=2&locale=EN-US&layoutCode=VSTY&pageId=1.1.1