The P 3I was an express locomotive whose special features were the unusual combination of inner and outer frames and internal cylinders. The locomotives had the wheel arrangement 4-4-2 and were initially powered by a two-cylinder saturated engine with internal cylinders. It arose from the requirement to be able to pull trains weighing 220 tonnes up gradients of one percent at 60 km/h and on the flat at 90 to 100 km/h.
With a coupled wheel diameter of 1,980 mm, they were certified for a speed of 100 km/h. The supporting framework was formed by a lightly built bar frame, which was additionally supported by an outer frame. The outer frame only started behind the bogie and included wheel housings for the coupled wheels. To reduce air resistance, the locomotives had a cone-shaped smokebox door and a wind-cutter cab.
In addition to the eleven engines built in 1898 and 1899, another one was built in 1904, which had a Pielock type superheater ex works. This could be installed relatively inexpensively around the existing heating pipes, but was not entirely convincing. So it was soon removed again and the locomotive, like the others, was regularly operated with saturated steam.
Due to the increasingly heavier trains, the engines were soon overwhelmed both in terms of traction and performance. In order to improve at least the second point, a complex conversion took place in 1913. Two low-pressure cylinders were mounted outside the frame, so the locomotives could now work as compound engines and the inner cylinders immediately served as high-pressure cylinders. As was to be expected, in addition to the increase in performance, this also resulted in a reduction in coal consumption of around 15 percent. The Reichsbahn took over five more and assigned them to the class 141. Since the subsequently converted powerplant turned out to be too maintenance-intensive, they were retired by 1926.