 Oslo Sporveier was highly satisfied with the SL79 articulated trams that had been delivered during the 1980s . In 1985 , work started on the development of a modified version for the western suburban lines . The stock on the Holmenkollen and Kolsås Lines were the first that needed to be replaced . At the same time , the company wanted to make the trains compatible with the metro so they could operate on both systems . The new stock was considered to be a trial ; if it met performance expectations , future orders could be made to replace the T1000 stock , after the latter reached the end of its economical life in about 2000 . The required specification for the new train was published in October 1988 . 
