sentences:  A severe coal shortage in June 1919 led to the service being cancelled altogether for six months, with the only passenger option a 12-hour-long journey by mixed train.When the "Express" returned in December 1919, it reverted to operating independently of the "Thames Express".The AB class locomotives were introduced to the "Rotorua Express" in 1925, and the superiority of these locomotives over prior ones as well as the raising of the speed limit to meant that the service took just 6 hours 40 minutes to run to Auckland and 10 minutes longer in the opposite direction.