Understandable, I used to take a K train twice every year before my grandparents passed away. I could always manage to get a seat as my route was less busy. It's certainly not a enjoyable experience.
However, I'd recommend to try again with the high speed trains. Chinese railway has changed a lot in the last 15 years. 5 hours between Beijing and Shanghai (that's more than 1200 km), on time, comfortable seats and quick transit to city center.
Or alternativly the sleeper trains, depart at night and get to destination in the morning, easily fits schedule and save one day of hotel. The comfortableness is a bit less than European's standard unless on the newest trains, though.
