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.