Tawa Flat deviation
[5] The original line from Wellington, constructed by the WMR, wound up the south side of the Ngaio Gorge with steep grades, tight curves, and tunnels with curves in them, to Ngaio, Khandallah, and Johnsonville before descending through difficult hilly country to Tawa Flat.
at Mt Misery The deviation was the first stage of the electrification of the NIMT railway from Wellington to Paekākāriki, and duplication to Pukerua Bay.
The grade in the opposite direction was 1 in 100 on the climb south from Tawa to Takapau Road and the north portal of No 2 tunnel.
The main contract for double track tunnels and bridges was advertised throughout the Empire but no satisfactory tenders were received, so the Public Works Department took over the whole job in May 1928.
[6] Work started from both portals, and also from a ventilation shaft with an entrance near the (superseded) Belmont Viaduct to Tunnel No 2.
Using a central pier in the middle of the Hutt road was cheaper but bought objections from motorists, and work was suspended for a time.
One meander on the former Ranui Golf Course known as the Devil's Elbow came close to the original railway line on what is now Duncan Street.
[18] With the completion of the deviation, the old single track railway line from Wellington to Tawa Flat was terminated at Johnsonville, electrified, equipped with a fully automatic electric signalling system to allow the automatic running of trains and renamed the Johnsonville Branch.
The first section of the Johnsonville-Porirua Motorway, which opened in December 1950, broadly follows the route of the former railway line from Johnsonville to Takapu Road.
The original structure, constructed with 212,000 superficial feet of kauri timber, was the largest wooden trestle bridge in New Zealand.
On 15 December 1951, after 14 years of disuse, the rusting 48-year-old steel viaduct was demolished for safety reasons by Territorial Force engineers as a training exercise using 44 kg of TNT.
[19] Some of the old route from Takapu Road to Tawa Street can still be found but parts of this section were destroyed by the Taylor Terrace housing development in the late 1950s.
The new deviation increased train speeds and reduced travel times between Wellington and Tawa, but curves south of No.
New railway stations on the deviation opened at Kaiwara (now Kaiwharawhara), Takapu Road, and Tawa Flat on the 19 June 1937.
Little used, Kaiwharawhara station was closed temporarily on the 13 June 2013 and permanently on the 21 November 2013 after an inspection revealed corrosion of the pedestrian overbridge.
2 Tunnel to serve Glenside, Churton Park and other expanding residential areas near Johnsonville, and underground at Newlands in No.
In 1925, the Merz & McLellan Report pointed out that electrification would remove the need to relieve the steep (1 in 57)[1] gradients to the Pukerua Bay summit by a deviation to the east, and was desirable in the planned long No.
From 24 July 1940, the line from Wellington to Paekākāriki was electrified at 1500 V DC overhead, being delayed by the late delivery of some items from England.
A miniature lever frame in the new Tawa Flat station building gave local control of the signalling to allow trains to be passed using the crossing loop, to control shunting movements, and to allow trains to terminated at Tawa Flat and returned to either Wellington or Porirua.
In conjunction with the completion of the double track from Tawa Flat to Porirua on the 15 December 1957, the capacity of the line was further improved with the installation of additional signals in No.
2 tunnel and between Takapu Road and Tawa Flat to allow trains to run at two-minute intervals between Wellington and Porirua.
Tawa Flat was now only staffed during the day from Monday to Friday or when local control of the signalling was required.
In conjunction with work on the upgrading of the HVDC Inter-Island link in the early 1990s, the 3.3 kV distribution from the northern portal of No.
The Tawa No 2 Tunnel and a short section of sharply-curved track north of Muri on the North–South Junction are the only sections of the Wellington overhead power catenary system where for technical reasons a modern auto-tensioned overhead system could not be installed; the Hutt Valley Line and the Johnsonville Line were upgraded in 2018-2021.
[27] Steam Locomotives hauled trains on the deviation between 1935 and 1940 but were generally prohibited from operation on the line after electrification due to the smoke nuisance in the tunnels.
With the completion of electrification in 1940, ED class locomotives, first introduced in 1938 for use on this line, were used to haul all goods and passenger trains between Wellington and Paekākāriki.
To provide additional capacity before the arrival of the Matangi multiple units, three EO class electric locomotives, originally purchased in 1968 for use in the Otira Tunnel, were transferred to Wellington in 2007 and with six SE class carriages operated services between 8 December 2008 and 10 October 2011, including a daily return service to Plimmerton.
The deviation is susceptible in places to flooding, slips, washouts, scouring of bridge abutments and piers, falling trees, fire, earthquakes and tsunamis, with the greatest risks in the section from Glenside to Takapu Road.
In the past, the line has been closed for short periods due to fires, flooding of the Kaiwharawhara stream, a burst stormwater pipe near the southern portal of No.
Frequent earthquakes in the Wellington area increase the risk of subsidence, slips, falling trees and structures, and track displacement.
