that do not traverse the length of the central section (such as pulp trains from Karioi mill) are entirely diesel-hauled to save switching the locomotives. Similarly, freight trains that are destined for or originate from the Marton - New Plymouth Line are diesel-hauled for 30 km section of the NIMT between Palmerston North and Marton. The locomotives had a number of teething issues when first introduced. After two years in service, two locomotives were withdrawn due to transformer failures, and converters were found to be unreliable and required replacing. The locomotives also suffered from longitudinal oscillation at low speed and required dampers to be fitted. This work was completed under warranty from Brush, after New Zealand Railways withheld the final payment of $44 million to Brush pending a solution to the locomotives' problems. As a result, fleet availability was poor, only 73% in October 1989, well below the target of 90% availability. The locomotives transformer issues continued, and an upgrade program was undertaken over 1993 and 1994 which saw the 20 remaining locomotives receive upgraded transformers and overhauled traction motors. This work was undertaken at the Hutt Workshops with EF30007 the first to be treated after having been out of action for some time prior. Legal disputes between Brush and New Zealand Rail (as it was by then) were settled in the early 1990s. Prior to the Stratford–Okahukura Line being mothballed usual operating practice was for the services originating from Auckland or Hamilton and Stratford to meet at Taumarunui and exchange trains there, with an EF regularly hauling the Hamilton-Taumarunui-Hamilton legs. With the mothballing of this line most services now run direct between Palmerston North and Te Rapa. Electric-hauled passenger services originated later, due to the then daylight NIMT service, the Silver Fern, utilising diesel-electric railcars. When the locomotive-hauled Overlander daylight service replaced the Silver Fern in December 1991, EF class locomotives began to be used to haul it across the central section, and later, the locomotives were used on the overnight Northerner passenger service until it was cancelled in November 2004. The EFs were not used initially on the Northern Explorer, KiwiRail experimented for a short while with towing the whole consist - including the diesel locomotive - over the central section. The need for the EF locomotives for freight services curtailed this. Despite the NIMT railway electrification and the Auckland railway electrification being built to the same voltage