Thursday, 25 April 2013

Port Chalmers-Dunedin-Mosgiel [3]

This is basically the “Dunedin Suburban Area”, which is the extent of coverage of suburban passenger transport in the Dunedin area. This is a three part article incorporating 33 maps which were originally produced at 1:2000 scale. N.B. Due to some vagaries of Blogger the maps get resized to a smaller scale so I can’t really help by saying what the actual scale is that you will see in the maps following.


Heading up through Green Island the Walton Park or Green Island branch went south off the main line, north/east of the station. The original route dating from 1874 when the line opened was deviated as part of the duplication works, although it’s not too obvious why such a large deviation was needed. Possibly the original junction was too close to Carnfort Street bridge, which I assume replaced a crossing, and perhaps also embankment construction was needed. Another possibility is that the later route already existed as a siding and in any case, perhaps it was just easier to make a new connection rather than lift the line up on the embankment like was done with the main line. I understand the latter route was kept as a siding after the rest of the line closed, and it got its own subway when the motorway was put through, which is still there today. As at other stations the main line was deviated through Green Island, this being an easier method of construction where there was enough space to deviate the main line. Where space was more confined, the original route was used with the tracks being lifted up in stages. The actual works carried out at Green Island were to “raise the dip” by which means the grade was levelled out.


Coming through Abbotsford there was again a main line deviation along with the duplication works. The Fernhill branch can be seen at the upper left of the map. The main works needed in the regrading of Abbotsford in 1911 were to lower the station yard by 18 feet (approx 5 metres).


Further south/west of Abbotsford can be seen a tramway at right angles to the track that apparently served a coal mine.


At lower right of this map is the Abbots Creek bridge. This bridge had to be raised in the duplication and the two sides were alternately lifted in stages with the abutments being extended, until the required height was reached (the embankments being built up in the same way and the bridges raised to match each embankment raising stage). The disused duplicate bridge at this location is still in place. Towards the middle/left of the map can be seen the original route leading to the original Chain Hills tunnel.


Here we see the tracks entering the two tunnels. The new Wingatui tunnel is about twice the length of the old Chain Hills tunnel. Below we see the tracks coming out at the south/west ends of the tunnels. The Chain Hills tunnel still exists today. The new double track tunnel was rather difficult to build costing 3 1/2 times as much per lineal yard as the Caversham Tunnel due to the nature of the ground that was encountered in building it. On reflection the conditions at Caversham were unusually good compared to a more typical experience of most railway tunnels in NZ, that Wingatui would tend to represent.


The site of Wingatui Station was changed to be closer to Dunedin and is directly adjacent to the tunnel portal.


Approaching Mosgiel the junction with the Outram Branch is encountered. Mosgiel (below) is the end of the double track section from Dunedin.