Tawa Flat deviation
The Tawa Flat deviation is a double-track section of the Kāpiti Line just north of Wellington, New Zealand, with two tunnels; the southernmost section of the North Island Main Trunk railway between Wellington and Auckland. It was built to bypass a limited capacity single track section of the original Wellington and Manawatu Railway line which ascended from Wellington to Johnsonville and then descended to Tawa Flat. The original name of Tawa Flat was changed to Tawa in 1959.
When opened to passenger trains in June 1937, the deviation reduced the travel time from Wellington to Porirua by 15 minutes, to 27 minutes rather than 43 to 48 minutes. By 2016, the time from Wellington to Porirua had further reduced to 21 minutes for stopping trains, despite extra stops at Redwood, Linden, and Kenepuru which each add 48 seconds to the travel time, and to 17 minutes for non-stopping trains.
The deviation
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.The new line split from the existing line at Thorndon, where the old line began its ascent to cross the Hutt Road toward Wadestown and Johnsonville. It shared tracks with the Wairarapa Line to the new Distant Junction, where Aotea Quay joins the Hutt Road, where it split from but followed the Wairarapa Line along the waterfront to Kaiwharawhara before climbing a new bank to enter the southern portal of the new No. 1 tunnel. From the northern portal of the new No. 2 tunnel at Glenside it followed the floor of the Tawa valley down to Tawa Flat where it joined the existing line south of McLellan Street, near the boundary of present-day Tawa College and Tawa Intermediate.
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. It saved, lowered the summit level from south of Raroa to at the north portal of No 2 tunnel, and reduced the maximum gradient from 1 in 36 and then 1 in 40 to a maximum of 1 in 110 in No 1 tunnel and 1 in 122 in No 2 tunnel. Previously the maximum load out of Wellington for a locomotive was 175 tons, and some trains were banked and then re-formed at Johnsonville. The new NZR Ka class steam locomotives could haul 600 rather than 280 tons, although further north between Plimmerton to Paekākāriki they were limited to 490 tons. Hence electrification extended to Paekākāriki, so including the climb up the bank between Plimmerton and 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.
Planning
In 1914 the NZR General Manager E. H. Hiley told parliament that the Johnsonville–Wellington section of the NIMT was approaching capacity, and in 1915 NZR district engineer A. Kock and the Public Works Department submitted a report which was deferred until after World War I.In 1923 various options were investigated including tunnels direct from Koro Koro to Linden and Petone to Tawa Flat. Five options for the deviation were considered. They were: two alternative routes with tunnels from Takapu Road to the Korokoro Stream near Petone, a tunnel from Takapu Road to a point a little southwest of the Horokiwi Stream on the harbour edge, an alternative northern portal for the northern of the two tunnels constructed, and the chosen route.
The total cost of the deviation was estimated at £1,409,000. Rock excavated from the tunnels was to be used for harbour reclamation to provide access to the first tunnel and to enlarge the rail yard in Wellington.
Construction
The deviation of about 7 miles as the Wellington-New Plymouth Line was authorised by the Railways Authorisation Act, 1924. The deviation was running generally to the eastward of the existing Wellington - New Plymouth railway. As built there were two long tunnels; Tawa No. 1 and Tawa No. 2. Tawa No. 2 tunnel, which remains the longest double track tunnel in New Zealand, passes under Newlands and comes out at Glenside, en route to Tawa.A preliminary contract to drive a header through Tunnel No 1 was let to Burnside and Matthews, who commenced work in July 1927. 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.
Work started from both portals, and also from a ventilation shaft with an entrance near the Belmont Viaduct to Tunnel No 2. The walls were of two foot concrete, and the tunnels were about high and wide.
The bank to the first tunnel was built in the harbour. Including the engine shed and goods yard, 68 acres of the harbour was filled in costing £490,000.
A "Hutt Road" flyover was constructed over the two tracks of the Wairarapa Line and the four lanes of the Hutt Road south of the Tawa No. 1 tunnel from the new roadway, Aotea Quay. As the flyover crosses them at an angle of 60 degrees it is long. 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. Originally a level crossing was proposed, but strong local objections led to the flyover being built.
The current configuration of railway tracks at the south end of Tawa No. 1 tunnel was adopted in 1965 when the down Wairarapa track was moved from the west to the east of the NIMT tracks and required the replacement of the turnouts at Wellington Distant Junction. This rearrangement was made possible by additional harbour reclamation during the construction of the Wellington Urban Motorway. The new layout reduced junction conflicts further south at Wellington Distant Junction near Aotea Quay and the Rail Ferry Terminal where the NIMT and the Wairarapa Line separate. Previously there were regular delays when trains crossed from one track to another at 30 km/h.
A bridge was constructed between the two tunnels to cross State Highway 1 where it commences an ascent of Ngauranga Gorge.
Six new railway bridges were constructed on the section of track from the northern portal of Tawa No. 2 tunnel at Glenside to just north of the Takapu Road railway station to allow the railway to cross the stream as it followed the stream down the narrow valley floor. From the last bridge, just north of Takapu railway station, to Porirua, the railway remains on the eastern bank of the stream.
After Takapu Road the valley widens into Tawa Flat where the stream meanders over the valley floor. 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. It was cut off and filled to allow for the construction of the new railway line and the new Tawa Flat railway station and rail yard.
Rowells Road, which crosses the railway over the northern portal of Tunnel No. 2, provides access to properties on the eastern side of the railway north of the tunnel portal. One house in the Willowbank area, just south of Takapu Road, was provided with an underpass to give access to the road. North of the tunnels, there are two road crossings. A bridge was constructed over the new railway at the south end of Takapu Road station to provide road access from the Main Road to the Takapu valley. This bridge now also provides access to the motorway interchange, Grenada North, and the supermarket on Takapu Island, an area between the rail and motorway. A level crossing was provided at Tawa Street to give access to Duncan Street and eastern Tawa.
Completion
The deviation opened to goods trains on 22 July 1935 using a single track, now the up main or northbound track, with tablet working. On 19 June 1937, the new Wellington railway station opened and the deviation opened to passenger trains using both the up and down mains with double line automatic signalling, providing double track running from Wellington to Tawa Flat. On the same day, the new stations at Takapu Road and Tawa Flat opened to passengers and the railway connection between Johnsonville and Tawa was severed.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. English Electric DM/D class electric multiple units entered service on the branch on 4 July 1938.
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 Belmont viaduct north of Johnsonville, between what is now Paparangi and Granada, could be seen a little to the east of the motorway. This was a steel viaduct, high and long, built in 1903 to replace the original viaduct completed in November 1886. 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. The original concrete abutments can still be found in the regenerating bush of Seton Nossiter Park.
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. Duncan Street in Tawa follows the route of the old line from Tawa Street to Tawa College. Widening of Duncan Street in the mid-1950s obliterated the remains of the old Tawa railway station platform, just north of the northern end of the present northbound Redwood platform, and other evidence of the former railway.
The new deviation increased train speeds and reduced travel times between Wellington and Tawa, but curves south of No. 1 tunnel, between the tunnels, and between the northern portal of No. 2 tunnel and Tawa prevent trains from maintaining full line speeds over the full length of the deviation.
On 15 December 1957, double track from Tawa Flat to Porirua was completed and additional signals installed between Kaiwharawhara and Tawa Flat. This further increased the capacity of the line and allowed more frequent services between Wellington and Porirua.