KBS 840, is an 88 kilometre long, electrified, double-tracked main line from Lichtenfels via Kronach to Saalfeld.

Climbing the ramps from Pressig-Rothenkirchen and Probstzella to Steinbach am Wald the line ascends the heights of the Franconian Forest (Frankenwald).

From there it switches to the valley of the Haßlach and follows it, getting ever narrower and steeper, to Pressig and onto the watershed between the Rhine and Elbe basins in Steinbach am Wald.

After passing the old halt of Lauenstein the line traverses the foothills of the Thuringian Slate Mountains on the left bank of the narrow river valley reaching Probstzella shortly after crossing the Bavarian-Thuringian border.

The railway accompanies the River Saale on the remaining ten kilometres to Saalfeld, crossing it once beyond Breternitz.