1990s. Due to a decline in rail freight volumes following land transport deregulation in 1983, and the introduction of electrification on the North Island Main Trunk, no new mainline diesel locomotives were introduced in the 1990s. Some second-hand locomotives were imported from Queensland Rail in the mid-1990s and rebuilt as DQ class locomotives, most of these locomotives were then re-exported to TasRail, then part-owned by Tranz Rail. The first new diesel locomotives imported to New Zealand since the 1970s were the DL class locomotives from China, arriving from 2010 onwards. Electric In the 1920s, two short sections of railway line were electrified at 1500 V DC: Arthur's Pass to Otira (electrified 1923) and Christchurch to Lyttelton (1929), both which have since been decommissioned. Electrification of the Wellington suburban network at 1500 V DC began in 1938 with the Johnsonville Line, followed by the North Island Main Trunk (NIMT) as far as Paekakariki in 1940, and the Hutt Valley lines in 1953–55. The NIMT electrification was extended to Paraparaumu in 1983 and to Waikanae in 2011. In 1988, 25 kV AC electrification of the North Island Main Trunk (NIMT) between Palmerston North and Hamilton was commissioned, and a new generation of mainline electric locomotives, the EF class, was introduced. In 2014, electrification of Auckland's suburban rail network was completed, also built to 25 kV AC. Couplings The Norwegian coupling, also called chopper coupler, was the standard coupler used in New Zealand for non-passenger rolling stock and locomotives until recent years. In the 1970s a large heavy-duty version of the chopper coupler was developed, initially for the DX class. Auckland's AM class and Wellington's FP/FT class "Matangi" electric multiple units use the Scharfenberg coupling. The Janney ("Alliance") coupler was first used on heavy coal trains and have been progressively introduced on newer rolling stock and rebuilt locomotives. A program of retrofitting older rolling stock has been underway since 2013. Workshops The New Zealand Railways Department had major workshops at Addington (Christchurch), Easttown (Wanganui), Hillside (Dunedin), Petone (Lower Hutt, near Wellington) then Hutt (Lower Hutt, near Wellington) and Newmarket then Otahuhu, (Auckland). All were progressively closed (mostly in the 1980s), leaving only Hutt Workshops still operating. A number of small maintenance depots also currently operate, for example at Addington, Christchurch. Accidents Safety regulation is the responsibility of the NZ Transport Agency. NZTA also investigates accidents and incidents with a view to ensuring regulations