The vehicles of the series 99.23 are from the German Reichsbahn procured meter gauge steam locomotives ( Neubaulokomotiven ). When they were classified in the operating park, they were given the company numbers 99 231 to 99 247.

To replace the partially outdated and moreover heterogeneous locomotive stock of Harz cross and Brockenbahn and narrow gauge railway Eisfeld-Schönbrunn procured the German Reichsbahn 1954-1956 a total of 17 Neubaulokomotiven. The design was derived from the unit locomotives of the DR series 99.22 . The Neubauloks are, however, consistently manufactured in welded construction, they have in place of the bar frame of the unit locomotive over a welded sheet metal frame , a more complex drive structure and fully welded boiler with mixing preheater.

The first seven units (99 231 to 99 237) were originally equipped with two Krauss-Helmholtz steering racks each formed from the wheelset at the respective locomotive end and the adjacent set of gear wheels. Because of problems with the sheet course was formed in the machines of the second lot but from the front wheelset and the first and second Kuppelradsatz three-axle Schwartzkopff Eckhardt steering racks . [1] [2] [3] [4] The locomotives of the first series were rebuilt according to the early 1960s and 1973/74 (as last 99 7231). On the initially weakened wheel flangesthe Treibradsätze one renounced later in favor of a better curve run entirely. Since only the third, so the Treibradsatz, and the fourth Kuppelradsatz are stored firmly in Lokomotivrahmen, a trackless wheel but can not undertake cornering work, since the locomotives no longer have a fixed wheelbase, but only a so-called guided length, which already by Richard by Helmholtz , the developer of the Krauss-Helmholtz frame, had been deemed theoretically appropriate.

The two-cylinder superheated steam engine with simple steam expansion drives the third set of gearwheels as a wheel set; the locomotives have Heusinger control with Kuhn's loop and initially springless pressure compensating piston valves of the type Müller , later converted to Trofimoff slide .

The frame of the Neubaulokomotiven used instead of the ingot frame of the Einheitslokomotiven was not as robust as the first, it occurred from the beginning cracks and bends, which led to difficulties in the conversation. From 2004 to 2010, therefore, ten locomotives (7232, 7234, 7236, 7237, 7239, 7240, 7241, 7243, 7245, 7247) were equipped with a new, structurally revised sheet metal frame and new steam cylinders in welded construction (The 99 7241 received the new cylinder already in 2001). 99 7235 is the only still usable locomotive with original frame at the end of 2016. All other locomotives are still available in the Harz, but no longer operational and z-placed mostly in smaller engine sheds on the routes .

After the decommissioning of the narrow-gauge railway Eisfeld-Schönbrunn , the four new steam locomotives used there (99 231, 99 235-237) also arrived at the Wernigerode site. But they had to be adapted to the Harzbahnen until the autumn of 1974 in the clutch and the brake. Due to the then existing large number of Neubauloks in Wernigerode, these were also used on a scheduled basis on the Selketalbahn . They relieved on the Selketalbahn the old Mallet locomotives 99.590 / 99 5906 offset there from the Harz cross and Brocken tracks .

All machines were equipped with oil main firing between 1976 and 1981 , because of the energy crisis in the GDR but 1982-1984 again reduced to coal firing. [5]They are the most efficient narrow-gauge steam locomotives that have been used on German routes so far. With a favorable overall efficiency of around 5.5%, they are also one of the most fuel-efficient narrow-gauge locomotives used in Germany. During test drives, it was found that the calculated boiler load of the boiler without combustion chamber of 55 kg / m²h could easily be increased to 68 kg / m²h. In order not to damage the boiler and engine in the long term due to excessive load, the performance charts were then only based on a heating surface load of 60 kg / m²h; At this boiler limit, the maximum effective power at the towing hook was 585 PSe at a speed of 15 km / h.

The eleven operational locomotives are now used mainly on the Harzquerbahn and Brocken traffic from Wernigerode and Nordhausen Nord. Their operating numbers changed from 99 231 ff. With introduction of the computer numbers 1970 in 99 7231 ff. And after the installation of the oil firing in 99 0231 ff. The HSB continues to use the series scheme of the German Reichsbahn of 1970.