IT IS 12:34 A.M. AND Metro's last Orange Line train pulls into the New Carrollton station right on schedule. The platform lights do their little dance one last time. As the last straggler snatches his fare card from the gate and carries it into the Maryland night, New Carrollton officially becomes the 60th and final Metro station to close tonight.

Inside, an eerie stillness falls over the station. The lights behind the Salem ads and the "You Are Here" map flicker like candles as the escalator continues noiselessly up from the deserted platform. A hush settles over the Metro system's rounded, churchlike vaults. Along the 60.5 miles of Metro track, silence rides the rails.

Within the hour, the tunnels echo shrieks from compressor engines in full throttle and the hoarse shouts of burly, flashlight-toting men. Like a team of worker ants creeping under the sleeping city, Metro's night crew, 87 strong, invades the tunnels. Tonight track workers will heave a giant 800-foot section of replacement rail into place at Stadium- Armory. They will replace by hand more than 100 corroded metal clips and fasteners along the elevated tracks at Rhode Island Avenue; and they will drive a smoke-belching monster called a track-grinder through sections of the Red Line. All of this will take place in the part of the Metro system most passengers only see passing by in a blur of repeating lights -- the dim tunnels between stations.

After the last passenger train clears the tracks, the third (or power) rail, which runs along the entire length of the system, is shut down near all the scheduled work sites. Other sections remain "live" throughout the night allowing trains to zigzag through the system for repair and storage.

f the third rail abound. Gene Cha, now a night shift supervisor, remembers "brushing" the third rail nine years ago. The 750-volt charge ran up his arm, across his chest and down through his other arm, which was grounded. "It felt," says Cha, "like someone hit me in the chest with a sledgehammer."

A small explosion often results at the point of contact with the powerful rail. Among the night crew, third rail wisdom has it that its 750 volts will "make a man's blood boil in three seconds," and then finish him off after six. As a warning to the crew, a bank of lights is routinely connected to the power rail with jumper cables at most work sites. When the lights are off, there's no power going through the rail. If the lights go on, it's show time.

s with electrocution, no Metro worker has ever been killed or seriously injured by the third rail. Several people, however, have committed suicide by tenaciously clutching it.

Through the night, test cars and a money- collection train run through the system. The biggest challenge for night supervisors is to keep workers alert during the early morning hours to prevent them from being hit by one of these trains.

Every oncoming train is preceded by a rumble, but not every rumble is an oncoming train. One of the most reliable calling cards of an arriving train is a subtle change in air pressure. First, the ears pop slightly, as they do in an airplane. Then, a puff of wind, slightly stronger and fresher than that usually pumped through Metro's extensive fan system, moves through the tunnel immediately after the change in air pressure. At that point, a 70 mph Metro train is about eight seconds away.

BY 1 A.M. ALL 12 WORK SITES have been set up inside the tunnels. On the elevated track leading up to the Rhode Island Avenue platform, workers have already begun the tedious task of replacing the metal fasteners. These are modern railroad ties that secure the rail to the concrete track bed. Eight fasteners every 10 feet, more than 250,000 throughout the entire system, every one of them slowly loosening under the weight of passing trains. When the fasteners become too corroded to be tightened, they are marked for replacement.The fasteners being removed tonight at Rhode Island Avenue haven't been replaced in 10 years.

"This is always the hardest bit, on a curve," says track worker Tom Walsh, jacking up one side of the rail as one would an automobile to change a tire. "You have to lift up the rail to get at the fasteners without cracking the rail."

When the rail has been lifted, the old fasteners are loosened and then pounded off with a furious sledgehammer attack. More than a few fasteners refuse to release their decade-old grip, sending sledgehammers still higher into the air. The work site begins to resemble an old photograph from the Reconstruction Era. In the last century many things have changed in rail construction, but the best way to knock off a stubborn rail fastener is still by hand, with a sledgehammer.

On the track opposite the fastener brigade, Metro's money train blurs past the work site, headed for a pickup at Union Station. The train moves from station to station under heavily armed guard, collecting all of those dollar bills from the fare card machines (Metro averages $400,000 a day in revenue, most of it in one-dollar increments). The train's final stop is in the middle of the tunnel just beyond the Gallery Place platform, underneath Metro's Fifth Street headquarters, alongside a metal vault built into the wall. The fact that the money train is usually protected by no less than a dozen guards armed with automatic weapons may explain why no one's attempted an underground version of the Great Train Robbery.

IN A TUNNEL BENEATH RFK Stadium, a dozen track workers grapple with the most difficult job of the night: replacing an 800-foot section of gently curved rail, which weighs more than 34,000 pounds. The old rail is lifted by attaching dozens of crank hoists to it with chains. Workers then put their shoulders into turning the cranks on the hoists, raising 17 tons of steel off the track bed.

This particular portion of track under the Stadium-Armory station has been in operation for eight years, and by now the wear on the edge of the rail has exceeded half an inch, the maximum tolerance allowed by Metro. More than half an inch of wear on a rail makes it difficult to provide the kind of flawless ride Metro riders have come to expect.

Lifting an 800-foot piece of rail into place (about half the length of the average station platform) can easily take all night, which is just what the track workers don't have. Their mandate is to clear the tracks by 5 a.m., and with rail replacement, there's no such thing as finishing the job tomorrow. If something goes wrong, every worker knows there will be a few thousand rush-hour passengers pacing the platform and banging on the kiosk window, demanding to know who's responsible for the delay.

Even 1,000 feet down the tunnel from the Rhode Island Avenue work site, the noise from the sledgehammers is painful, as the workers bash away at the bolts and fasteners. Sounds are ridiculously magnified in the tunnels -- a friendly toot on an airhorn from a flatbed bores through the eardrum. Even gentle sounds take on a new meaning. The trickle of water from the street into the tunnels -- a constant problem throughout the system -- sounds like gushing, underground streams.

Over at Gallery Place, a gigantic track grinder is roaring through the station, scraping tiny corrugations off the rail. Like a rolling knife sharpener, the track grinder smooths the edges of the rail, shooting blue-orange sparks from 24 grinding stones onto the platform and against the tunnel walls. The sound made by 24 grinding stones in one of these tunnels is roughly equivalent to a few thousand finger- nails scratching against a giant blackboard. Even worse than the noise are the acrid particles of metal suspended in the air after the big machine has passed. Although some metal shavings are sucked up by exhaust fans, much of the stuff stays in the tunnel, clinging to clothes and equipment. "One night, we had the exhaust fans on manual when the grinder went through," recalls a supervisor. "I was blowing that stuff out of my nose for a week."

BY 4 a.m., most of the men at the work sites are approaching the last leg of their assignments. The track grinder has moved through the Red Line all the way down to

"Judy," Metro's affectionate name for Judiciary Square. The fastener replacement project at Rhode Island Avenue, the station cleanings at Court House and Rosslyn and the guardrail adjustments at Silver Spring are all on schedule and nearing completion.

At Van Ness-UDC, five workers stand in the track bed with shovels in hand as a section of rail is heated to 2,000 degrees, the temperature necessary for welding. The track may look like an unbroken ribbon of steel from the platform, but on closer inspection, the rail is laid out in sections, welded together and ground smooth so no trace of a break can be detected. The average length of a rail is a quarter of a mile, but small sections of replacement track, like this one, dot the system.

The platform here at Van Ness looks like a giant tool chest -- a dozen sledgehammers, three oily brooms, and an array of muddy shovels are laid out over the floor. During revenue hours, it's rare that a platform floor is allowed to become even dusty. Metro has spent the last 10 years raising cleanliness from a virtue to an immutable law. After the track workers finish tonight, a team of custodians will clean up, restoring order in the temple.

Several workers lean against shovels, ready to put gravel on flying sparks from the welding compound as it's heated. "That stuff spews out like lava sometimes and sticks to you," says track worker Ira Blossom, who is keeping his distance. When the rail and the welding compound each reach 2,000 degrees, the molten compound flows into the break in the rail like mercury. "There we go, baby," says one worker, "Now we're cookin'."

The finishing touches on the weld are complete. Equipment is dragged off the platform and piled on top of a flatbed car. Some workers ride the flatbed "home" to the Shady Grove yard; others pile into a Metro pickup truck for the trip back to New Carrollton. Flatbed cars roll through the tunnels to yards in Brentwood, Alexandria and West Falls Church.

Only the flatbed near the rail replacement job underneath RFK Stadium is still waiting, 15 minutes before the crew has to be off the tracks. The new rail has already been lowered into position, leaving a team of inspectors to check the placement with fold-up yardsticks. At last, a thumbs-up signal passes down the tunnel. The last work site begins to break up.

Most workers from the Stadium- Armory site take the flatbed back to the yard. After spending four hours pounding away in a tunnel, a slow flatbed ride can be downright therapeutic. The car gently rocks from side to side as the stations pass, as in a dream. Deanwood . . . Cheverly . . . Landover . . .

The intercom crackles with the voices of train operators checking in with Metro's central control for the morning shift. "Roger, Operator Dixon," the voice over the intercom says." There are no restrictions on the railroad today . . . "

The railroad. As much as the streamlined cars and dancing lights would have you believe otherwise, Metro is, after all, a railroad. Its public face is all granite and chrome, but its soul is steel.

As the flatbed bringing the night crew home creaks into the New Carrollton yard, dawn breaks over the trees. The first passengers of the morning are already standing outside the locked station gate, reading their papers. They are oblivious to the track workers, who struggle home to bed through the early mist.