I've never travelled the North Island Main Trunk in daylight, but my parents did it recently and were disappointed because all the great engineering was beneath them and therefore not visible from the train! I do know a couple of spots on the main highway where you can see some viaducts. I'm sure more can be found by going off the beaten track a bit. As an indicator of the current place rail holds in New Zealand, the Overlander almost ceased operations a couple of years ago. We're still a nation obsessed with trucks for some reason.