Discovering the old Atlanta is a lot like going on an archeological expedition: you have to know where to dig and what to look for - and it pays to have a lively imagination.

Unlike such other historic Southern cities as Charleston, S.C., or Savannah, Ga., Atlanta offers no dramatic vistas of white-columned, antebellum mansions or live-oak-canopied, Colonial squares to make its past come easily alive.Union Gen. William Tecumseh Sherman, a man still remembered here as being a little too fond of fire, reduced most of the city of "Gone with the Wind" legend to ashes during the Civil War. Since then, successive generations of builders and developers have been no less sparing of the past, razing landmark after landmark in the name of progress.

But if historical sleuthing appeals to you - and if you have an archeologist's knack for envisioning the whole from the merest part - then a search for the old Atlanta amid the new Atlanta's glittering skyscrapers and fast-paced freeways can provide unending hours of adventure and discovery.

Although young in age as Southern cities go, Atlanta boasts a rich and varied past. Born as a railhead in the north Georgia wilderness in the 1830s, the town known successively as Terminus, Marthasville and Atlanta quickly grow into a bustling crossroads of travel and commerce in antebellum Dixie.

During the Civil War, it earned undying glory as the "Citadel of the Confederacy," serving the Rebel cause as a strategic railroad hub, supply depot and hospital center. Sherman, in fact, complained that Atlanta did more to keep the war going than any other Southern city excepting possibly Richmond, the Confederate capital.

Here, to initiate you into the joys of exploring its colorful heritage, is an introductory walking tour that carries you on a quest back in time to Atlanta's 19th century railroad era, one of the most dramatic and eventful periods in the city's history.

The trail winds through a roughly six-square-block area around the downtown state capitol and can easily be contracted or expanded to fill as much of a spare morning or afternoon as you care to give it.

The starting point, appropriately enough, is the Zero Milepost, Atlanta's oldest surviving man-made monument and - despite its rather unimpressive appearance - one of the city's most hallowed historic treasures.

It is located by the side of the tracks under the Central Avenue viaduct. To reach it, take the elevator inside the public parking garage (on the viaduct) marked "90 Central Avenue" to the train level and follow your nose to the tracks.

A small, well-worn white stone obelisk, the milepost was erected in 1850 to mark the southern terminus of Atlanta's first railroad: the Western & Atlantic, a 138-mile line created by the state to link north Georgia with Chattanooga, Tenn., then a gateway to lucrative "western" agricultural markets.

Today, the only trains leaving from this spot are the restored vintage passenger coaches of the New Georgia Railroad. Run by the state as a tourist attraction, the line offers day-time excursions each Saturday on a history-filled loop around the city and to nearby Stone Mountain, one of Atlanta's scenic wonders.

Just next door to the milepost is another prized landmark from Atlanta's 19th century railroad age. It is the Georgia Railroad Freight Depot, a one-story, red-brick building that is Atlanta's oldest surviving building (or part of a building: a three-story, cupolaed office tower that once graced the front was destroyed by fire in 1935).

Built in 1869 for the then princely sum of $35,000, its dedication four years after the Civil War was an occasion of immense civic pride for a city still struggling to recover from the wartime devastation.

A series of walking tour markers will help you find your way to the spots of historic interest. Among the key attractions are the five-story Block Building (built in 1882), which once housed a candy factory on its upper floors; a portion of the original facade of the Hotel Jackson (1892), one of the old Atlanta's most popular hostelries.

One of the 50 street lamps erected by the Atlanta Gas Light Co. in 1856 also may be seen here, close to the spot where it originally stood. The hole in its base was left by a Yankee cannonball during Sherman's siege of Atlanta.

At the southeast corner of Central Avenue and Martin Luther King Jr. Drive, stands Atlanta's second-oldest surviving building: the red-brick, Gothic-style Shrine of the Immaculate Conception, which was erected in 1873.

The original church, a one-story wooden structure, was built on this site in 1848. Father Thomas O'Reilly, a priest appointed to serve there in 1861, earned a niche in Atlanta's pantheon of heroes for persuading Union commanders to spare his church and four others that once stood in this vicinity from fire when Atlanta was torched.

"If you burn the Catholic church," he warned, "all Catholics in the ranks of the Union Army will mutiny."

The final stop on this journey back in time is the state capitol itself, a Neoclassic-style building whose dome has been a landmark feature of Atlanta's skyline for more than a century.

Built of Indiana limestone and finished inside with Georgia marble, the building was formally dedicated July 4, 1889.

The gold overlay on the dome, however, is of more recent vintage. It was applied in 1958 with gold from the north Georgia town of Dahlonega, an early 19th century "gold rush" site.