In October 1997, construction of an upgraded Nationaltheatret Station started and was taken into use on 16 December 1999. The work expanded the station from two to four platforms, thus increasing the applied capacity for the whole tunnel from 16 to 24 trains per direction per hour. The expansion cost NOK 920 million, which included blasting of rock and pouring of concrete. The work resulted in of new tunnel, a new entrance hall and art. The station was designed for 40,000 passengers daily.