The line became part of the North Island Main Trunk when the government bought the WMR in December 1908.

Trains going north often used banking engines on the steep grade from Lambton to Ngaio and sometimes when going south from Johnsonville to Raroa.

[3] The line became a branch when the Tawa Flat deviation of the NIMT opened to passengers in 1937, and was sometimes called The Hill (in NZR jargon).

The livestock were originally driven on foot through Johnsonville streets from a stockyard adjacent to the station, but after protests sidings near Raroa were opened on 2 February 1958.

The first through revenue-earning train on the line ran on 30 October 1886 from Longburn to Johnsonville, carrying 355 sheep and 60 head of cattle for the Wellington Meat Preserving Company at Ngauranga.

[8] 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.

[9] 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.

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[13] and metres[14] are: 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[15] and the WCC Strategy & Policy Committee[16] 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;[17] this required enlarging the tunnels and increasing platform clearances and lengths.

Construction took place after 20:00 on Sunday – Thursday nights to minimise disruption to commuters, with services being replaced by buses.

A steel section of the pedestrian overbridge at Raroa railway station was also replaced in Easter 2022 (16-18 April)).

Buses were added to supplement the remaining service capacity pending the planned introduction of the FT/FP "Matangi" EMUs on 19 March 2012.

[27] The new Matangi EMUs struggled on the Johnsonville Line with rail track squeal and to maintain the schedule used for the English Electrics, especially in wet conditions.