I was at the Centenary of the opening of the Otira Rail Tunnel recently.
The Construction cost was apparently £1.5 million. Using the reserve bank inflation calculator from around 1919 (centre date of the 10-year project) and taking it to today’s prices gives us $180 million. That’s an 8km tunnel through terrible rock and weather. Makes you wonder why some piddling tunnel (or is it all three of them) under Auckland harbour is going to cost $45billion!