Tawa Flat deviation facts for kids

The Tawa Flat deviation is a special double-track railway line near Wellington, New Zealand. It's part of the Kapiti Line and the southernmost section of the main railway line that connects Wellington to Auckland, called the North Island Main Trunk (NIMT). This section is about 8.38 miles (13.49 km) long and includes two tunnels. It was built to create a faster, easier route for trains.

Before this new line, trains had to use an older, single-track line built by the Wellington and Manawatu Railway Company. This old line went up and down steep hills from Wellington to Johnsonville and then down to Tawa Flat (which is now just called Tawa). The new deviation made train travel much quicker. When it opened in June 1937, it cut the travel time from Wellington to Porirua by 15 minutes! By 2016, the trip was even faster, taking only 21 minutes for trains that stop at all stations.

Why the New Railway Line Was Built

The original railway line from Wellington was quite difficult for trains. It twisted and turned through the Ngaio Gorge with steep slopes and tight curves, even inside tunnels. This made it hard for trains to go fast or carry heavy loads.

In 1914, railway managers realized the old line was getting too busy. They needed a better way to move trains. After World War I, they started looking into different ideas for a new line. They thought about several tunnel routes, but finally chose the route we see today.

The new line was planned to be much straighter and flatter. It would save about 2.5 miles (4.0 km) of track and lower the highest point trains had to climb from 518 feet (158 m) to just 195 feet (59 m). The steepest slopes were also made much gentler, from 1 in 36 to about 1 in 110. This meant trains could carry much more weight. For example, the new NZR Ka class steam locomotives could pull 600 tons, compared to only 280 tons on the old line.

Building the Tawa Flat Deviation

Building this new railway line was a huge project! It was officially approved in 1924. The plan included two long tunnels:

• Tawa No. 1 Tunnel: 1,238 metres (4,062 ft) long
• Tawa No. 2 Tunnel: 4,323 metres (14,183 ft) long (This is still the longest double-track tunnel in New Zealand!)

Work began in July 1927. The government's Public Works Department took over the main construction in May 1928. Workers dug the tunnels from both ends and also from a special ventilation shaft for Tunnel No. 2. The tunnel walls were made of thick concrete.

A big part of the project involved filling in 68 acres (28 ha) of Wellington Harbour to create new land for the railway tracks and a larger rail yard in Wellington. This reclaimed land also helped provide access to the first tunnel.

A special bridge, called a flyover, was built over the Wairarapa Line tracks and the Hutt Road. This bridge, 48.8 metres (160 ft) long, helped trains avoid crossing busy roads at ground level. Another bridge, 67 metres (220 ft) long, was built between the two tunnels to cross State Highway 1.

North of the tunnels, six new railway bridges were built. These bridges allowed the railway to cross the stream as it followed the valley floor. The new line also changed the landscape in Tawa. A bend in the stream called "Devil's Elbow" was cut off and filled in to make space for the new Tawa Flat Railway Station and its rail yard.

Opening and Changes Over Time

The Tawa Flat deviation first opened for goods trains on 22 July 1935, using one track. On 19 June 1937, the new Wellington railway station opened, and the deviation started carrying passenger trains on both tracks. This meant trains could run more often. On the same day, new stations opened at Takapu Road and Tawa Flat (now Tawa). The old railway connection between Johnsonville and Tawa was then closed.

The old line from Wellington to Johnsonville was still used, but it was shortened and became the Johnsonville Branch. It was also updated with electric trains.

Over the years, the railway line continued to improve. In 1957, the double track was extended from Tawa Flat to Porirua, which allowed even more trains to run.

Railway Stations on the New Line

The new deviation bypassed all the old stations between Wellington and Johnsonville. New stations were built for the new route:

• Kaiwharawhara Station: This was an existing station on the Wairarapa line, but it was updated for the new deviation. It had two island platforms. However, it was closed permanently in 2013 due to safety concerns with its pedestrian bridge.
• Takapu Road Station: A new station with separate platforms for trains going in different directions.
• Tawa Railway Station: The original Tawa Flat Station was on a different street. The new Tawa Flat Station was built on an island platform in the valley. It was a busy station because it was the end of the double track for a while. In 1959, its name changed to Tawa Railway Station.
• Redwood Station: An extra station called Redwood Station was opened in 1963 between Takapu Road and Tawa. This helped serve new houses being built in the area.

There have also been ideas for new stations at Glenside and even underground at Newlands inside Tunnel No. 2. Many stations now offer "park and ride" facilities, where people can park their cars and take the train into Wellington.

Electric Trains and Signalling

One big reason for the deviation was to make it easier to electrify the railway line. Electric trains don't produce smoke, which is great for tunnels! In 1933, it was decided to electrify the line from Wellington to Paekakariki. This was finished in 1940. Even today, the inside of the Tawa tunnels still has a thick layer of soot from the steam trains that ran there before electrification.

The new double-track line also got a modern signalling system. This system uses lights and special circuits to tell trains when it's safe to go, allowing them to run much closer together. This greatly increased how many trains could use the line.

Over time, different types of electric locomotives and passenger trains (called multiple units) have been used on the line. From the NZR ED class and EW class electric locomotives to the DM/D and newer EM class and Matangi multiple units, the trains have changed to provide better service.

Safety and Risks

The Tawa Flat deviation is a strong railway line, but it faces some natural challenges.

• Flooding: Parts of the line, especially near the Kaiwharawhara stream, can flood during heavy rain. The stream is very close to the tracks in many places.
• Earthquakes: Wellington is in an earthquake prone region. The railway line crosses the Wellington Fault and runs between other major fault lines. This means there's a risk of ground movement, slips, or damage to the tracks during an earthquake. After big earthquakes, the line is closed for safety checks.
• Tsunamis: The section of the line near Wellington station and Kaiwharawhara is close to the harbour, so it could be affected by a tsunami.

Engineers have done work to reduce these risks, like building dams to control floodwaters and clearing vegetation to prevent fires. Despite these risks, the Tawa Flat deviation has been a very important and safe railway line since it opened in 1935.