M42 (sub-basement)

[5] However, the new Grand Central LIRR terminal is in areas deeper,[5] descending more than 175 feet (53 m) below street level at its lowest point.

The former coal bunker space, located above the ventilating room and right below street-level, now houses a backup battery, moved from the 50th Street Plant.

In a nearby underground facility just to the north, Substation 1B provides DC power to the Graybar Building and Grand Hyatt.

[5] According to a reporter for The Poughkeepsie Journal, the staircase is inconspicuously placed in a public corridor and dug straight out of the underlying bedrock.

The first such plant, built for Grand Central Depot in the 1870s, stood in the surface-level railroad yards at Madison Avenue and 46th Street.

[14][15]: 150  The two-smokestack 50th Street Plant could supply a daily average of 5 million pounds (2,300,000 kg) of heating steam.

[7][15]: 152  In addition, the Grand Central power plant provided steam and hot water to nearby buildings.

The surrounding buildings had no basement space for boiler rooms, as they stood above Grand Central's rail yard.

[19] According to author Sam Roberts, the facility was featured in a navy training film as the safest place in New York during a nuclear strike.

[5] Roberts also notes that the space was removed from the building's floorplans, and contains a red button that can shut down the railroad.

Operation Pastorius, a plan by Nazi Germany to disrupt rail lines and destroy manufacturing sites and infrastructure, was staged and foiled in 1942.

There is no direct evidence that M42 was a target, though the saboteurs did meet at the station's information booth and newsreel theater, and M42's entrances were patrolled by armed guards during this time.