The station is sandwiched midway between the street-level entrances to the modern Broadway Station above and the platform below, accessible through a silver door marked “T personnel only.’’ Beyond the old mosaic “Broadway’’ sign at the platform, a track bed and tunnel extend about a quarter-mile to a street-level fence that faces Foundry Street, across from an MBTA rail yard.

“If you were standing here in 1919, you’d be taking a streetcar to Andrew or to Bayview’’ in South Boston, Clarke said at the old Broadway station.

But on a walk through this ghost world, it’s easy to imagine the bustling Boston of old, when streetcar traffic dominated daily life and was so robust that an underground system, the first American subway, was needed to relieve congestion above.

As the group stepped through the rubble, flashlights and strung-together utility bulbs lighted the old corridors like a noir movie, casting eerie shadows over dust clouds. Smells varied by location, with a sewage leak penetrating the mildew at one point, and cool air freshening segments near the portals. MBTA walkie-talkies, brought along for safety, bounced a cacophony of chatter against otherwise silent walls.

Clarke, a 65-year-old who became interested in transit when he was 10, probably knows as much about the Massachusetts Bay Transportation Authority’s past as anyone. He identified about 10 noteworthy sections of the abandoned tunnel system at the Globe’s request. And on a recent weekend, he joined a rare expedition into a hidden world that includes locations unfamiliar even to many longtime MBTA employees.

“It’s all here and it’s exactly as it was built,’’ he said. “Exactly.’’

Bradley Clarke, president of the Boston Street Railway Association, stands in a cobwebbed tunnel that, from 1897 to 1962, linked the present-day Boylston Street Station with the South End. He takes measure of the nooks on the sides of the tunnel walls that were designed to fit 19th-century track workers, smaller men by contemporary standards, and stares at the rusted tracks.

This hidden world of abandoned tunnels and stations lies below the familiar landmarks: the Freedom Trail, City Hall Plaza, and Brattle Square in Cambridge. But the corroded rail spikes, the antiquated mosaic signs, and the dust-covered corridors offer another layer, a Pompeii of sorts, preserved under the civilization that followed it.

Boston’s bars have poured their last drinks. Stragglers are stumbling home. But beneath the modern metropolis, in the bowels of the city, a group of six men wanders abandoned tunnels with heavy coats and flashlights, dodging the occasional rodent, to uncover the lost relics of America’s first subway system.

Over the years, crews have come through the old Tremont Street tunnel to run utility lines or, in recent years, to consider and then reject the possibility of fitting high-speed Silver Line buses in the narrow tunnels. There is also a large mound, probably 15 feet high, of rusted-out 10-gallon water and biscuit containers that date to the 1960s, the height of the Cold War, when the tunnel was considered a nuclear fallout shelter.

Its use declined and it closed for good in 1962 after the last of those trolley lines was converted to a bus route, a phenomenon that occurred earlier and to a greater extent in other cities around the nation as Americans adopted cars and the suburban lifestyle that came with them.

“This was the Boston roller coaster,’’ Clarke added, noting the steep decline on the walk down from Boylston station to the South End. “The fun lasted until about 1961.’’

But between 1897 and 1901, it would have been bursting with trolleys, as many as 100 per hour, Clarke said.

The old Tremont Street Subway segment from Boylston Station to Eliot Norton Park, which extends for about a quarter-mile, connected the first section of the subway, now part of the Green Line, and as many as 20 trolley lines, Clarke said.

The old tunnels and tracks in the subway system were abandoned over decades for a variety of reasons. In some cases, the MBTA or its predecessor companies built extensions that eliminated the need for a subway turnaround loop or train storage yard. Other tunnels were rerouted to make way for station updates, the abolition of overhead rail lines, or changes in the city’s layout.

Because some abandoned tunnels overlap with the modern subway system, the MBTA insisted that the tour take place overnight on a weekend, when the modern-day tunnels are largely free of trains and maintenance crews.

Mulhern, who began at the T as a track walker and has rebuilt many of the existing tracks, helped lead the Globe’s tour. He was joined by two MBTA employees - Charles Murphy and Barry Page - assigned to keep the group safe while walking near live electrified tracks.

Though used only between 1917 and 1919, the abandoned station became something of a test kitchen for the subway system. In the 1930s, Boston Elevated Railway Co. even tried growing mushrooms there, Clarke says. In the 1980s, the T brought blind passengers in to test rubber warning strips now used on platform edges. And after the terrorist attacks on Sept. 11, 2001, firefighters simulated a burning train emergency in the tunnel, says Michael Mulhern, a retired MBTA general manager.

At the end of the tunnel, near the Church of All Nations at Tremont and Charles streets, Mulhern locates a broken hatch leading to the sidewalk. It opens just a crack, enough to glimpse the fluorescent lights of the modern South End.

By the 1980s, the T was phasing out tunnels for another reason, the rerouting of the Orange Line and the removal of the elevated track that extended over Washington Street into Roxbury.

A few hundred feet from the current Chinatown station, the old Orange Line tunnel that once connected to an overpass above the Massachusetts Turnpike ends abruptly. Built between 1904 and 1908, it was sealed off in 1987, when the elevated line was taken down.

“Here we are 101 years later, looking at what is essentially a reasonably complete subway,’’ says Clarke, standing amid discarded cinder blocks and a junked cash register.

Other elements of the old subway have been reclaimed for new uses. The old Court Street Station, used between 1904 and 1916 to connect with what later became the Blue Line, sits atop the modern Blue Line, adjacent to Government Center, formerly Scollay Square.

It is difficult to locate, up an iron stairway and hidden behind two 4-foot-high doors, like the kind in Willy Wonka’s chocolate factory. And even once inside, it is barely recognizable as a station, now that much of it has been cleaned, refurbished, and converted into an attic used for electrical utilities and ventilation.

Clarke is one of the few people who knew about it, and is excited to have found it again after at least 20 years. “See the curvature of the subway. It’s all here,’’ he says.

Likewise, an old track loop near Harvard Square has been largely cleaned up since the 1980s, when it was last in use. To get to the old, gritty section requires confronting a door with a sign that reads “Danger,’’ next to a handwritten warning: “Do not let this door close behind you. You will be locked inside.’’

Once past the daunting signs, the group enters a darker and dirtier room, then crosses a puddle over a wooden plank. A few hundred yards later, Mulhern finds a staircase that leads to an exit on the street, across from an International House of Pancakes.

The final stop on the tour leads to East Boston’s Maverick Square Station and the search for a trolley loop that was abandoned on Jan. 5, 1952, another casualty of the end of the streetcar era. The MBTA recently finished a complete renovation of the station, and the group had received mixed messages about whether the loop might have disappeared when crews expanded the station.

But in the back of a power substation, through another hidden door, lies a gravel path the length of one or two city blocks.

There are no tracks. The walls are bare and musty. There are puddles. It does not look much like the narrow Green Line tunnel witnessed earlier in the evening or the shadow station at Broadway. But there are signs it was once used as a subway: the curvature of the tunnel walls and the cutout niches every 10 feet that would have provided a haven for rail workers who needed to escape an oncoming trolley.

Clarke, who had harbored his own doubts about the ghost tunnel’s existence, is thrilled. It’s about 4:30 a.m. and he’s been up all night, but is not tired at all. This is a transit historian’s dream.

“My God,’’ he says. “It’s still here.’’

Noah Bierman can be reached at nbierman@globe.com.

© Copyright 2009 Globe Newspaper Company.