extension (the "mountain line") to Mariazell, a compound steam engine and a superheated steam engine were acquired as further developments of the U series. In 1906, the Mariazell extension was sufficiently complete for freight traffic to be worked through to the terminus. On 2 May 1907, passenger service to Mariazell began running. That same summer, the final main line extension to Gußwerk was brought into service. For the mountainous extension to Mariazell, an especially high-performance engine was needed. The Krauss locomotive works in Linz proposed to build a locomotive with four powered axles and a tender, four of which were built by 1906 and used superheated steam. They were designated Mh (nowadays ÖBB 399). In 1907, two locomotives with compound steam workings were acquired and designated Mv. The "h" stood for "Heißdampf" (superheated steam), and the "v" for "Verbundantrieb" (compound working). Since the latter locomotives did not prove their worth, the next order was for two further locomotives of the Mh series. Since many passengers were expected, a great number of four-axled passenger coaches were bought, which were comparable in comfort and accoutrements with contemporary standard-gauge coaches. Also in 1906, three bigger and stronger steam railcars were delivered. The "Lower Austrian-Styrian Alp Railway" (Niederösterreichisch-Steirische Alpenbahn) as the railway was known in Austro-Hungarian officialese, was thereby complete. Far-reaching plans for an extension over the Styrian Seeberg and a connection with the likewise narrow-gauge Thörlerbahn, and thereby with the Styrian railway network, had no work done on them owing to the outbreak of the First World War. Also, a connection to the Ybbstalbahn was never built. Once open, the rush of passengers was so great that for a time, the railway did not even bother with advertising. Among the various kinds of freight carried on the railway were agricultural products, ores from local mines, and wood from the heavily forested mountain region. Wood remained the most important kind of goods on the Mariazell Railway right up until freight operations were discontinued. As early as 1909, standard-gauge goods wagons were being transported along the Mariazellerbahn on transporter wagons, insofar as the railway's narrow loading gauge would allow it. Electrification It quickly became apparent that the series Mh and Mv locomotives were not adequate for the traffic on offer. Several scenarios for raising the railway's performance were considered, among them double-tracking and the acquisition of an even stronger type of steam locomotive. At this