The route to the southeast removed a hundred miles from existing railroad routes to the southeast coast. And it was built so straight and with such a gentle grade that one big engine with a long train could haul as much as three trains on the existing railroads. But to cut across the Allegheny Mountains with low grades and minimum curves was expensive. To cross the Blue Ridge Mountain at Altaps, NC, an 8-mile loop that climbed 300 feet was built to reach the second longest tunnel in the railroad. It was more than a half mile long. (More on the loops.) The longest tunnel is through Clinch Mountain at 4,200 feet.
To support long heavy trains with minimum maintenance, bridges were minimized by using extensive cuts and fills. Nonetheless, they did build six significant bridges, the highest being the Copper Creek viaduct. In addition to bituminous coal throughout the area, the Blue Ridge section "is one of the richest in iron, copper, mica, kaolin, manganese, limestone, and timber in the whole Appalachian region, if not in the whole country."
The railway operated in five states, crossed four mountain ranges and five major watersheds, included fifty-four tunnels (totaling almost 10 miles) and 17,000 feet of bridges. [TennesseeEncyclopedia]
Because it connected the Midwest with the Southeast, about a third of its revenue was from bridge (merchandise) traffic, but its main function was to haul coal. Not only did it have coal mines on its own route, it provided a more direct route out of the mountains for coal that came from interchange with other railroads in the area. The details of the engineering of the line are: "Actual maximum grades were kept at 1.5% against Southbound traffic through the Breaks of the Big Sandy River, 1.2% up the Blue Ridge climb Northbound, and less than a mile of 1.8% Northbound grade at Sandy Ridge. Maximum curves of 14 degrees were not exceeded, while most were 10 degrees or less in 20 degree country." [AppalachianRailroadModeling]
In 1924 it was reorganized as the Clinchfield Railroad Corporation and leased to the Atlantic Coast Line and the Louisville & Nashville railroads for 999 years. Both of these railroads are now part of CSX. [Carolana]
"It made record profits for almost ninety years." [AppalachianRailroadModeling] An interpretive sign claims an average of 18-24 coal trains pass over the route. But I can't find a date on that sign. As of 2015, CSX has stopped using this route because Allegheny coal production has so dramatically declined.