Locomotives with no coupled axles were soon no longer sufficient on the Grand Duchy of Baden State Railways either, and so class XII was developed with a 4-4-0 wheel arrangement and a remarkably short wheelbase. Certain similarities to Swiss locomotives are not surprising, since the Swiss network was also used in part.

Earlier locomotives often had a 2-2-2 wheel arrangement, but when adding a second coupled axle, a front bogie was chosen instead of two individual carrying axles. In order to be able to continue to use the existing turntables, the overall wheelbase was designed to be as short as possible. Due to the large driving wheels, this was not achieved simply by moving the bogie back; the bogie itself had to be made very short and the distance between the remaining axles kept as short as possible.

Since development was making rapid progress at the time, the engines differed depending on the year of construction. It was noticeable that the steam dome was relatively small in the first batches and significantly larger in the later ones. What they all had in common was that the external leaf springs were noticeable and the deceleration was carried out via spindle brakes on all wheels of the tender. Due to the design of the boiler and the position of the cylinders in front of the bogie, a speed of only 60 km/h was possible despite the large wheels.

Despite the inadequacy, the engines of the series designated as type III from 1868 became indispensable, so that from 1869 a reinforced version was procured as III a. From 1881, many of the 41 examples of the XII or III and the III a were converted to the III b with an even more powerful boiler and longer frame. The locomotives that had not been converted were taken out of service at the beginning of the 1890s, and the rest were able to survive for a few more years after the turn of the century.