The Main North Line between Christchurch and Picton was built in two main sections starting from the respective termini. Heading north from Christchurch it was known as the Cheviot Section north of Waipara, whilst south from Picton it was called the Picton Section. It was not until December 1945 that the last linking track was opened allowing the entire route to transport through traffic.
Between Seddon and Vernon, the line passes over the Dashwood Pass. This is well known to railwaymen as a steeply graded and sharply curved, hilly section of track. There are several large horseshoe curves and the whole has to be traversed at a slow speed due to gradients and tight curvature. In this section the first two tunnels of the Picton Section were constructed, No. 1 (MNL No. 23, approx 50 metres long) at the commencement of the climb up the Dashwood Pass, and No.2 (MNL No. 22, about 70 metres long) near the summit. This part of the route was opened for traffic in October 1902.
Tunnel No. 22 in the Dashwood Pass was the site of a fatal rail accident in May 1966 when an overnight Picton to Christchurch passenger train oversped and derailed on the downgrade from the summit of the Pass. The leading locomotive hit the tunnel mouth and was severely damaged, killing the driver and seriously injuring the secondman. The accident resulted in additional safety equipment being fitted to the DG class locomotives which were being deployed in the South Island during that era.
Tunnel 23 was daylighted in 1979 and Tunnel 22 in 1981. The tunnels were subclearance for modern rolling stock.
The photo below shows the remains of the wall of Tunnel 22 which are still in place at its site, and the map shows its location in the Dashwood Pass.
