Barcelona to Madrid.
Barcelona has less than 2 million population. There's f**k all in between. They run heaps of HSR trains between the two cities daily.
On the surface that sounds great, but lets take a closer look at the numbers for this.
There is only 600km between Barcelona and Madrid (I'd say it's more comparable Sydney to Albury)
Spain fits twice the population of Australia into something the size of NSW.
Catalonia (the region Barcelona is in) alone has a population base about the same size of NSW.
Greater Madrid has a population base around 7 million
The two regions between Catalonia and Madrid, Castilla–La Mancha and Aragon, the f**k all in between as you put it, have a combined population of well over 3 million.
For competition there is well under 20 flights per day in each direction. In comparison, Sydney to Melbourne by plane has at least 50 flights per day and is the top five busiest air routes in the world.
Would the cost to get a fast train between Sydney and Melbourne ever be cheaper than flying?
To run high speed rail between Sydney and Melbourne, and make it cost effective, you'd need at least 5 Canberra's in between.