If you are wanting a train from Auckland to Hamilton, I would imagine it would need new lines. For it to be a goer, I think it would have to be a pretty fast train around 200 kph, something in the region of 30-40 minutes.

It would be awesome for Hamilton and I'm pretty sure that Hamilton would do quite well out of it with businesses considering being based in Hamilton. However, given all of the half arsed kiwi infrastructure projects that have been done over the last 100 years, I don't see this anywhere on the horizon.