At the turn of the century steam engines still pulled passengers into Manhattan, and every train that came and went from Grand Central Terminal had to make its way through the narrow, choking Park Avenue tunnel. Smoke and fumes were worst at rush hour, and they were particularly bad on the morning of January 22, 1902. A New Haven commuter train was stopped at a red signal in the tunnel when a New York Central express came plunging through the murk on the same track. Seventeen people died, and the state legislature passed a law that as of July 1910 no steam locomotive could enter Manhattan on a passenger line.

One result of this ruling was a whole new Grand Central Terminal. It is one of the glories of New York, a structure so splendidly designed that no amount of tampering over the years has managed to diminish the sense of luxury and purpose it sheds on those who hurry through it. When the megalomaniac slab of the Pan Am Building was dumped on top of the station in the mid-1960s, Grand Central effortlessly absorbed the twenty-five thousand additional people poured into its corridors daily.

A quarter-century earlier Grand Central had also handled the immense tidal flow of American troops going and coming from the greatest of wars. Few of the millions who passed through the tall rooms on their way to the ends of the earth knew that deep beneath their feet other soldiers were guarding the heart of the station.

“They were very careful about it all through the war,” says Dan Brucker, who handles press relations for Metro North (the more streamlined but less resonant name of the old New York Central). “Anybody who got in there could cripple troop movements throughout the whole Northeast.”

The carefully guarded object was a bank of ten General Electric rotary converters, hundred-ton monsters that have been delivering the current that has powered the trains that come and go from Grand Central since 1916.

“They’re about ten stories down,” says Jim Frawley as our open-cage elevator drops below the concourse. “It’s the largest and deepest railroad substation in the world.”

Frawley is supervisor of substations. A big man, built on a scale with the machinery he manages, he started in Grand Central thirty-two years ago as a plumber. “It’s all been on-the-job training, you could say.”

The door opens and we step out into a room that looks every bit as big as the station far above it. Vast as it is, it does not dwarf the line of dynamos, each rising perhaps fifteen feet above the concrete floor. The machines have such immense, iron authority that it takes a moment to realize that only two are spinning, and that they occupy only a fraction of the room.

“There are four left,” says Frawley. “We’ve already replaced the others.” He gestures to a neat gray row of metal cases that look like a line of lockers in comparison with the behemoths whose place they have taken. “These two are still working, but they’ll be gone by the end of the summer.”

The dynamos have earned a rest. “They don’t owe us anything. They’ve been in service twenty-four hours a day almost since the station opened.” Frawley points: “Numbers five and six there are the two largest rotary converters ever built for railroad use.”

What they convert is alternating current into direct. “Con Ed sends us eleven-thousand-volt, twenty-five-cycle power. We’re almost their last customer who uses twenty-five-cycle power. Today it’s all sixty-cycle.”

The power comes in through oil switches—big circuit breakers—into the transformers that stand behind the dynamos and reduce the voltage from 11,000 to 480. “Then it’s fed into the dynamos through those AC rings. It goes through the field”—he gestures to the spinning coil—”and it’s collected off the DC commutators on this side.”

The power is collected by the brushes, pure carbon plugs with two braided copper shoelaces trailing off them. The copper “pigtails” brush the electricity off onto the collector ring. There are six to seven hundred brushes on each generator, and they must be changed every few weeks.

From the collector ring the electricity passes through knife switches big as baseball bats onto the copper bus that feeds the direct current out into the third rail that powers the trains.

Will Frawley be sorry to see his dynamos go? “Sure, in a way. But you know how it is—no spare parts. Anything goes wrong, it’s bubble gum and baling wire. The only parts we still get from outside are the brushes.

‘The people who make those, they’ll be sorry to see ‘em go.”