An 8 hour train ride is outside of what is acceptable for normal train use. Up to about 5 hours on the train most people will prefer the train to flying. For short and medium distance trips trains have several advantages. The train is probably closer to your house and where you are going (air ports are way out on the edge of town in most cases, while train stations are closer to the center). You don't have the long wait for security for the train. You get more legroom on the train. For longer trips an airplane is worth those disadvantages, but not for shorter trips.
>>bluGil+(OP)
Yes, but without an airport in my wife's hometown, 8 hours by HSR is better than flying from Beijing to Changsha or Guangzhou and transferring to HSR.
Chinese HSR stations can be as inconveniently located as airports, so that isn't much of a benefit. Security is a bit better, they mostly make you put your bag through some sort of X-ray machine that I doubt they are looking at.