I know both Hels and Diane B love special train trips and so do I, not that I have done much else but high speed trains. The list of trips around the world one could take must be very long. Within Australia we have a few quite special trips.
But across the Tasman Sea are two very different but what would surely be very memorable trips. Both are on the South Island of New Zealand. The Coastal Pacific runs from Christchurch to Picton at the very north of the island and for a good part of its journey hugs the east coast with wonderful sea views. You can read here for a decent description of the trip by Hels.
I am not sure which I would chose as the TranzAlpine sounds fantastic too. From Christchurch in the east it climbs over the mountains to the west coast town of Greymouth. The line does require clearing of snow at times though. Here is a high quality and well filmed video taken only a month ago. Driving blind is unimaginable to me. Whoooooosh!