PARIS — An elderly couple waltzes to the steady carnival music of an organ grinder on the dingy platform of a Paris subway station. But it's nearly 3 a.m., and the station has been closed since World War II.

They are part of a traveling nocturnal party that barrels through the tunnels of the Metro long after the passenger trains have been tucked away for the night. These riders are on an exclusive, all-night Metro history tour, organized a dozen times a year by a group of railway buffs.

The ADEMAS association, dedicated to restoring old trains and preserving the Metro's history, has been around since 1992. (ADEMAS stands for Association D'Exploitation du Material Sprague; the Paris Metro's Sprague-Thomson model cars date to the early 1900s.)

But until recently, the group was as mysterious as the phantom stations they visit on their tours.

Rumors about the all-night Metro rides circulated by word of mouth and in Internet chat rooms, but details were almost impossible to track down — until June, when ADEMAS launched its Web site. But even that lacks specific times and tour dates.

Association president Julian Pepinster says it's with good reason. There are just 2,400 spots available each year.

"There's a constant demand for the tours," he said in an interview. "Even with a dozen tours a year, there's always a six-month wait."

It begins before midnight down a cobblestone side street just outside Paris' northern border. A small crowd, jittery like children off to summer camp, gathers behind an iron gate that opens onto a vast rail yard: a messy stack of rail ties, wood and machinery.

"Tickets! Tickets, please!" a man calls from a rickety table.

The guides, railway enthusiasts from ADEMAS, explain historical changes in voltage, and how different types of wheels connect to the rails. (If a rubber tire goes flat, there's a smaller iron wheel behind it that will take over.)

Some people take notes, some take photographs, and many glance over their shoulders at the large clock in anticipation.

It's almost midnight. The Metro is about to close, clearing its tunnels of trains and making way for the tour. The night travelers clamber on to the brilliantly green Sprague-Thomson car, a vintage model from 1930.

On board, there are polished wooden bench seats and enamel decoration in cream, mustard and burgundy. Original maps showing Line 2 stops and streets are posted overhead, and iron baskets hang above the seats for small luggage.

A sign above the seats informs passengers that smoking and spitting are prohibited — a nod to modern-day mores.

After the Metro's creation for the Universal Exposition of 1900, lines spread throughout the city. The insignia decorating the doors of this train bear the initials CMP, one of the two Metro companies that competed for customers before World War II.

The train departs with a jolt that surprises a few passengers. They smile nervously and grip the seat backs, laughing as high-pitched whirring and chugging noises accompany the train into the city's underground tunnels.

From the loudspeakers, a comfortable banter soon emerges from the night's hosts, veritable tomes of history on the Paris Metro. Whenever the train is moving, for the next five hours, it's nonstop Metro trivia.

"We started off as a small group of friends," Pepinster explained, adding that the association includes Metro employees and retirees, police officers, insurance salesmen and research scientists.

The train travels through the city center in between the last two passenger trains, called the Sleeve and the Sweeper.

Bewildered late-night Metro riders watch from the platform as the 1930s train rumbles through the station without stopping. Many step back and wave at the tour passengers, who enthusiastically reciprocate.

After changing Metro lines via a shortcut normally only used by repair cars, the Sprague heads to its first stop: Croix-Rouge in southwestern Paris, one of the Metro's "phantom stations."

The economic stresses of World War II forced Paris to close many Metro stops. Much of the population had been moved to the front lines, and the city needed to conserve energy.

Four of the stations that closed in 1939 — Arsenal, Champ de Mars, Saint-Martin and Croix-Rouge — never reopened. After the war, the two Metro companies combined to form the RATP (the agency that handles local public transportation, Régie Autonome des Transports Parisiens). The RATP decided those four stations were unnecessary because of other stations nearby.

Everything is oddly familiar as the train pulls into the grime- and dust-covered platform of the abandoned station at Croix-Rouge. Like other, still-operating stations designed in the early 20th century, Croix-Rouge has tile-covered vaulted ceilings and large frames for advertisements. A rectangular space announces the station's name in white on blue.

Only this station is covered from top to bottom in filth and graffiti.

If the defacing of an important piece of Paris' history is reason to mourn, tonight's passengers are oblivious. The doors open, spilling almost 200 people onto the platform, and the 20-minute party begins.

The organ grinder wakes up from his cushy spot on board and begins cranking out music and singing along. People rush the bar in the first-class car for coffee or beer. Some dance. Others explore the station's fenced-off stairwells and hallways.

Soon the guides usher the crowd back on board, and the group is off to other phantom stations. At a stop around 3:30 a.m. at a training station for Metro drivers, the group gets champagne and pastries.

The evening that started out so jovial is winding down to a quiet halt in western Paris. The guides provide a breakfast of hot chocolate and croissants before the weary crowd heads home, via the first Metro of the new day.