The line is
single track through very steep terrain rising 150 m above sea level in its 10 km length, with the highest point (152 m) at the north end of Kaka Tunnel. The
ruling grade is 1 in 36, with long sections from Wellington to Crofton Downs and Khandallah of 1 in 40. There are seven narrow tunnels, six bridges, and three
passing loops. There are three
level crossings with half-barriers; at the Simla Crescent and Khandallah stations, and at Fraser Avenue between Khandallah and Raroa stations (the Fraser Avenue crossing was last to get barriers as well as bells, in 2009). There is a private rail crossing to a house immediately south of the Fraser Avenue crossing; and a pedestrian crossing from Burma Road to Poona Street, Khandallah between the Khandallah station and the Rangoon Street road overbridge. Pedestrians can also cross the line at the Ngaio station. The signals were operated by the trains, a standard SLA Absolute Block System; the first train at a crossing station or siding (Wadestown, Ngaio and Khandallah) claimed the next block (if two arrived simultaneously, the
up train got priority as there was a five-second delay for the
down train). The system was revised in July 1969; the second platform siding at Johnsonville was removed and the goods yard (since removed) became a switch-locked siding. Signal boxes were initially retained at Ngaio and Khandallah. Up to 1984 when the line was truncated and the station was moved to accommodate the mall, the station had two sidings on either side of the station platform and the preferred platform was alternated every 12 hours to equalise usage. The Wellington City Council let a $1.7m tender to replace the Rangoon Street single-lane overbridge of c1906, which crosses the Johnsonville line, with a two-lane bridge. Work commenced in June 2008 and was completed by December 2008. Three traction substations along the line take electricity from
Wellington Electricity's 11,000-volt distribution network before transforming and rectifying it to 1500-volt direct current for the overhead traction lines. The substations are located at Wellington (shared with the Kapiti and Hutt Valley Lines), Ngaio and Khandallah. The names of the tunnels and their length in feet and metres are:
Upgrade in 2008–2009 The North Wellington Public Transport Study by
GWRC and
WCC considered four options for improved public transport: enhanced rail; bus on street; conversion to a
guided busway; and conversion to
light rail. On 16 November 2006 the GWRC Public Transport Committee and the WCC Strategy & Policy Committee accepted a "Do Minimum" option involving retention of the line and replacement of the current
DM units with the same number of refurbished
EM/ET class (Ganz Mavag) units; this required enlarging the tunnels and increasing platform clearances and lengths. GWRC now only use
Matangi units on this line after the decision to withdraw all Ganz Mavag units in the region from service in favour of a larger order of Matangi units. GWRC envisaged (2007) that the track through the tunnels would need to be lowered by 120 mm, depending on the new units. Lengthening of passing loops and platforms was also likely to be needed, and the estimated cost was $5 million. A programme of preparatory work for the tunnel upgrading commenced on 7 September 2008 and was completed in February 2009. Construction took place after 20:00 on Sunday – Thursday nights to minimise disruption to commuters, with services being replaced by buses. The seven tunnels were upgraded in January 2009 during a period in which the line was closed to all traffic. The work included: • lowering the track and widening the side clearances in the seven tunnels • lengthening the three crossing loops, allowing longer trains • upgrading platforms by lengthening them and increasing clearances • increasing clearances under two bridges: Lower a rail bridge in Ngaio Gorge (between tunnels) and lower the level of the track under the
Raroa Station footbridge • new power substation at Ngaio
Upgrade in 2019–2022 Upgrading of the overhead power catenary system with replacement of the remaining one hundred wooden poles by steel poles was completed in 2022. A steel section of the pedestrian overbridge at
Raroa railway station was also replaced in Easter 2022 (16-18 April)). == New rolling stock ==