Tawa Flat deviation

The NIMT Tawa Flat Deviation's double track climbing towards Tawa No. 1 tunnel over the up Wairarapa Line and Hutt Rd, with Ngauranga station in the background alongside the State Highway 1 motorway.

The 8.38 miles (13.49 km)^[1] Tawa Flat deviation is a double-track section of the Kapiti Line just north of Wellington, New Zealand; the southernmost section of the North Island Main Trunk Railway (NIMT) between Auckland and Wellington. It was built to bypass a section of the original Wellington & Manawatu Railway line which ascended to Johnsonville and then descended to Tawa. The deviation saved 2.5 miles (4.0 km), lowered the summit level from 518 feet (158 m) to 195 feet (59 m) and reduced the maximum gradient from 1 in 36 (on the initial climb out of Wellington) and then 1 in 40 (to Ngaio) to a maximum of 1 in 100 (in the opposite direction, on the climb south from Tawa).^[1]

The time to Porirua was reduced by 15 minutes, now 27 rather than 43 to 48 minutes. 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 Paekakariki they were limited to 490 tons.^[2] ^[3]

The total cost of the deviation was estimated at £1,409,000. The bank to the first tunnel was built in the harbour. Including the engine shed and goods yard, 68 acres (28 ha) of the harbour was filled in costing £490,000.^[1]

The original name of Tawa Flat had been changed to Tawa in 1959.^[4]

The deviation was authorised in 1924. It includes two long tunnels, Tawa No. 1 (1,238 metres (4,062 ft) with a 1 in 110 gradient) and Tawa No. 2 (4,323 metres (14,183 ft) with a 1 in 122 gradient).^[5] No. 2 tunnel passes under Newlands and comes out at Glenside, on the way to Tawa Flat. The double track tunnels were constructed by the Public Works Department as no satisfactory tenders were received. Work started in May 1928, from both portals and also from a ventilation shaft with an entrance near the (superseded) Belmont Viaduct. The walls were of six inch (nearly 610 mm) concrete, and the tunnels were about 6 metres (20 ft) high and 7.8 metres (26 ft) wide. ^[2] ^[6]

A flyover was constructed over the two tracks of the Wairarapa Line south of the Tawa No. 1 tunnel, and a bridge between the tunnels which crosses State Highway 1 which ascends the Ngauranga Gorge.

The current configuration (as shown in the photo at right) was adopted when the down Wairarapa track was moved to the east when the Wellington Urban Motorway was constructed. This also removed junction conflicts further south.

The deviation opened to goods trains on 22 July 1935 and to passengers on 19 June 1937.^[7]

The line from Wellington to Paekakariki was electrified at 1500 V DC from 24 July 1940, being delayed by late delivery of some items from England. Initially locomotive-hauled trains were used, and DM/D electric multiple units first ran on the line on 5 September 1949.^[8] As was pointed out in the 1925 Merz & McLellan Report, electrification removed 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 long No. 2 tunnel. ED class electric locomotives were introduced in 1938 for use on this section. The electrification was extended further north to Paraparaumu in 1983 and then in 2011 to Waikanae.

External links

1927 article with graphs

References

1 2 3 4 Railway Magazine February 1934 pp. 100-105
1 2 Murray 2014, p. 151.
↑ Murray and Parsons 2008, p. 33.
↑ Carman, A. H. (1970, also 1956, 1982). Tawa Flat and the Old Porirua Road. Wright and Carman. p. 235. Check date values in: |date= (help)
↑ Murray and Parsons 2008, p. 34.
↑ Murray and Parsons 2008, p. 29.
↑ Murray and Parsons 2008, pp. 30,33.
↑ Murray and Parsons 2008, p. 36.

Rails through the Valley: The story of the construction and use of the railway lines through Tawa by Bruce Murray and David Parsons (2008, Tawa Historical Society) ISBN 978-0-473-14410-4
Murray, Bruce (2014). A History of Tawa. Wellington: Tawa Historical Society. pp. 150–153. ISBN 978-0-473-25848-1.
Pierre, W.A. (Bill) (1981). North Island Main Trunk: An Illustrated History. A.H. & A.W. Reed. pp. 179–190. ISBN 0-589-01316-5.

New Zealand railway lines

Main lines

North Island	East Coast Main Trunk North Island Main Trunk (inc. Kapiti Line)

South Island	Main North Line and Main South Line (inc. Lyttelton Line), known together as the South Island Main Trunk

Secondary lines

North Island	Marton–New Plymouth Line North Auckland Line Palmerston North–Gisborne Line Stratford–Okahukura Line Wairarapa Line (inc. Hutt Valley Line)

South Island	Midland Line Nelson Section Otago Central Railway Stillwater–Westport Line Waimea Plains Railway

Branch lines

Upper North Island	Cambridge Dargaville Donnellys Crossing Glen Afton Kinleith Kumeu–Riverhead Manukau Mount Maunganui Murupara Newmarket Okaihau Onehunga Onerahi Opua Rotorua Taneatua Thames Waiuku and Mission Bush

Lower North Island	Ahuriri Castlecliff Foxton Gracefield Greytown Johnsonville Melling Mount Egmont Moutohora Ngatapa Opunake Raetihi Taonui Te Aro Waitara Wanganui

Upper South Island	Blackball and Roa Cape Foulwind Conns Creek Eyreton Fairlie Ferrymead Little River Methven Mount Somers Oxford Rapahoe Rewanui Hokitika/Ross Seddonville Southbridge Waiau Waimate Whitecliffs

Lower South Island	Bluff Catlins River Dunback and Makareao Fernhill Hedgehope Kingston Kurow Moeraki Mossburn Ngapara and Tokarahi Outram Port Chalmers Roxburgh Shag Point Tapanui Tokanui Tuatapere Waikaia Waikaka Wairio Walton Park Wyndham

Private lines

Significant proposals

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