That's the south island midland line over Arthur's Pass? That route looks and has always looked like it must have been obscenely expensive to build, likely why it took so long. The first mental image that comes to mind of the Cristchurch-Greymouth route is of Kb's hauling long strings of those dinky 4 wheel coal wagons. I suppose more recently it'd be DX's heading things up instead. Better that way. Running steam through those long tunnels doesn't seem like my idea of a good time. There's one video posted on Youtube that shows a NZR engine slipping inside a slick/wet tunnel and the cab pretty much immediately fills with steam.