In 1988 a major reshuffle of the mainline locomotive fleet took place nationwide as a result of the opening of the NIMT electrification and introduction of the EF class to handle the central section. For the DX class, this meant leaving the upper North Island and being split into two fleets for service between Wellington, Gisborne and New Plymouth, and one in Dunedin. This was the first use of the class on the Palmerston North - Gisborne Line and the northern section of the Wairarapa Line between Masterton and Woodville. At the same time, experiments began with DX class locomotives in the then electrified Otira Tunnel. While the initial tests were unsuccessful, later testing in 1991 found that modifying the locomotives with special air intake ducts and adding ventilation fans would allow the DX class to be used without electric locomotives.