The viaducts that lifted Atlanta above its railroads

Why it matters: Downtown's viaducts helped fuel Atlanta's growth after the Civil War and created a two-level cityscape that stands to this day.

Driving the news: "The Elevated City," a new Georgia State University Library exhibit, revisits the bridges' histories and how they shaped Atlanta business, transportation and culture.

Catch up quick: In the late 1800s, Atlanta leaders struggled to keep up with the city's rapid boom after the war.

• From 1880 to 1930, according to GSU researchers, Atlanta's population mushroomed from under 4,000 people to 130,000.
• Viaducts, city leaders decided, could help pedestrians, automobiles and trolleys bypass railroad crossings completely.

Zoom in: Well into the 1920s, the city constructed elevated spans at Washington (now Courtland), Broad, Forsyth, Mitchell streets and a grand viaduct linking Peachtree and Whitehall streets, among others.

Yes, but: Business owners near the viaducts faced a conundrum: The street-level storefronts were now underground, and some moved their businesses to the second floor.

• Some of the city's most historic streets were effectively abandoned for decades. One viaduct, according to the exhibit, "put a roof" over one of Atlanta's red-light districts.

The intrigue: In the 1960s, a GSU art professor, architect and a city commission hatched the idea of turning the historic streets into a nightlife district called — what else? — Underground Atlanta.

• The viaducts and the idea of capping the railroad tracks inspired future visions, including an architect's bold "City Beautiful" plan and contemporary projects like The Stitch, Centennial Yards and the Georgia World Congress Center's plaza.