Tuesday, 24 October 2017

Otago Central Railway [42]: Tiger Hill Overbridge (Bridge 74a)

I have decided to keep going with station diagrams for the time being but it will be a busy time with writing Part 2 and finishing off the maps needed for Part 1 so we will see how that goes. Omakau is the next station, followed by Lauder, Auripo, Ida Valley, Oturehua, Wedderburn and then Ranfurly to bring Part 2 to completion. It is more the case of having all the detail to hand to fill in Part 2 which looks like it will be appropriately divided at Ranfurly, and then a couple of months later Part 3 will cover the remainder back to Wingatui, although I don't have as much detail about the line before 25 miles because I don't have the chainage charts for that part at present.

Between Omakau and Chatto Creek on the Otago Central Railway the principal geographical feature of the railway is a climb out of Omakau to the top of the Tiger Hill, via which the line then descends to Chatto Creek. Tiger Hill is noted for its sweeping horseshoe curves which take the line 180 degrees one way and then back the other way to carry on in more or less the same direction as the early part of the descent. Chatto Creek is more or less at the foot of this steeply graded 1 in 50 section which is about 6 km long according to the curve and gradient diagrams (which I will place online sometime soon).

The top of Tiger Hill is marked by what was originally a level crossing just past the 114 mile peg. Because the original crossing of the road would have been over a cutting when the railway went through, the road was altered to make a level crossing nearby. The overbridge No. 74a was built in 1964 and straightened the road up. As there are many level crossings on the Otago Central railway it must have been desirable to have this overbridge installed, perhaps because of problems with visibility at the level crossing, however it is also possible realigning the highway was a significant impetus. There were in fact several overbridges built for the line and some of the others have previously been looked at and like this one we will revisit them with the aerial photography coverage that is available similar to that published below.

Overview of Tiger Hill. Chatto Creek station is upper centre, and Bridge 74a is lower right.
Bridge 74a at the top of Tiger Hill, also showing the former road route. These days there is no trace of the former level crossing on the ground.
A 1938 aerial photo showing the level crossing.
 The bridge in 1965 a year after completion, and the old road and crossing clearly visible.
 From the highway as it is today, the old road probably went through the where the gate is on the right.
 The old road as seen from the current highway bridge heading towards the level crossing.
Site of the level crossing on the ground.