alt Hacker NewsChina has invested into high-speed rail for the same reason that Japan, France and Spain did: the big cities tend to be in the "sweet spot" distance of 250-750 km from one another, which is very comfortable to ride in a high-speed train.
Distances over 1000 km are already favorable for passenger jets. Even in today's Western Europe, there just aren't that many really long distance trains (by which I mean something like Amsterdam-Barcelona or Copenhagen-Milan).
Beijing - Shanghai is about 1200 km. HSR has a 70% market share. The trip takes 4.5 hours.
The only real reason you would fly this route would be as a connecting flight. But if you just want to travel from Beijing to Shanghai, HSR is the obvious choice. It probably takes less time overall, and it's way more comfortable. You can even order takeout from restaurants along the way, delivered to your seat.