In 1865, the parliament of Württemberg determined to build the Black Forest Railway from Stuttgart via Leonberg and Weil der Stadt to Calw.
Planning and construction of the line was directed by Carl Julius Abel.
The Black Forest Railway was planned and built as a main line railway, that is with few curves, large radius curves and few level crossings.
Moreover, the civil engineering of the line—the two tunnels and the bridgedecks of all the bridges—was designed to allow the construction of a second track.
The formation of the line was however generally built as a single-track line, except for the Althengstett–Calw section, which was built as two tracks, so that operations on the 10.5-kilometre-long Calw–Althengstett ramp could run smoothly.
This was the first two-track section built on a line in Württemberg during its initial construction.

The Royal Württemberg State Railways completed the section from Zuffenhausen to Ditzingen in 1868 and a year later the section to Weil der Stadt.
The last and hardest part of the line between Weil der Stadt and Calw was not completed until 1872.
It included a tight loop with a radius of 335 metres to pass by Hackenberg (hill) near Schafhausen.
A loop built on a similar principle was built at the end of the line at Hirsau.
Here it was necessary to overcome the large height difference between Calw and Althengstett by extending the line.
The line runs through the valley of the Tälesbach, a tributary of the Nagold in an extended loop and then turns back and runs through the Hirsau Tunnel to the flank of the Nagold valley.
That meant that trains running from Calw towards Althengstett, initially ran to the north and then looped around on the eastern slope of the Nagold valley to run parallel on the same hillside but a lot higher and now running south.
The model for this type of alignment was the Brenner Railway in Austria.

Apart from the Hirsau Tunnel, the planners considered it necessary to create two more tunnels in this last phase of construction due to the topographical conditions, but only one was built, the 696-metre-long Forst Tunnel.
The other, which was originally to be built between Althengstett and Calw, could not be built because of geological problems.
Instead, the Feldhütte cutting, which was 1,150 metres long and up to 38 metres deep was built.

The Calw maintenance shop (Bahnbetriebswerk Calw) was completed at the end of the track in Calw in 1872.
In 1865, the parliament of Württemberg determined to build the Black Forest Railway from Stuttgart via Leonberg and Weil der Stadt to Calw. Planning and construction of the line was directed by Carl Julius Abel. The Black Forest Railway was planned and built as a main line railway, that is with few curves, large radius curves and few level crossings. Moreover, the civil engineering of the line—the two tunnels and the bridgedecks of all the bridges—was designed to allow the construction of a second track. The formation of the line was however generally built as a single-track line, except for the Althengstett–Calw section, which was built as two tracks, so that operations on the 10.5-kilometre-long Calw–Althengstett ramp could run smoothly. This was the first two-track section built on a line in Württemberg during its initial construction.
The Royal Württemberg State Railways completed the section from Zuffenhausen to Ditzingen in 1868 and a year later the section to Weil der Stadt. The last and hardest part of the line between Weil der Stadt and Calw was not completed until 1872. It included a tight loop with a radius of 335 metres to pass by Hackenberg (hill) near Schafhausen. A loop built on a similar principle was built at the end of the line at Hirsau. Here it was necessary to overcome the large height difference between Calw and Althengstett by extending the line. The line runs through the valley of the Tälesbach, a tributary of the Nagold in an extended loop and then turns back and runs through the Hirsau Tunnel to the flank of the Nagold valley. That meant that trains running from Calw towards Althengstett, initially ran to the north and then looped around on the eastern slope of the Nagold valley to run parallel on the same hillside but a lot higher and now running south. The model for this type of alignment was the Brenner Railway in Austria.
Apart from the Hirsau Tunnel, the planners considered it necessary to create two more tunnels in this last phase of construction due to the topographical conditions, but only one was built, the 696-metre-long Forst Tunnel. The other, which was originally to be built between Althengstett and Calw, could not be built because of geological problems. Instead, the Feldhütte cutting, which was 1,150 metres long and up to 38 metres deep was built.
The Calw maintenance shop (Bahnbetriebswerk Calw) was completed at the end of the track in Calw in 1872.
Black Forest Railway (Württemberg)