In 1874, the Marietta and North Georgia Railway began constructing a line that would eventually connect Atlanta with Knoxville. By the time the route was completed in 1897, the railroad was known as the Atlanta, Knoxville and Northern Railway. However, one weakness in the route was a set of switchbacks just north of the Georgia-Tennessee border.
In 1898, the railroad replaced these switchbacks with a double loop around a hill called Bald Mountain. In 1902, the Louisville and Nashville bought the railroad, and eventually issued this postcard. Though abandoned for freight service in 2001, the Tennessee Valley Railroad operates tourist trains over the loop.
This route was once called the Hook & Eye Line, with the loop being the eye and a nearby 180-degree turn being the hook. The hook was bypassed in the 1950s, so the eye today is usually called the Hiwassee Loop after the nearby Hiwassee River.
As the above aerial from Google maps shows, the mountain the loop goes around is no longer bald, and it is difficult to see the loop from any view except straight overhead. However, I hope to take the line next time I get down to Tennessee.