I would not sit on a train for 20 hours either but while Chinese airlines have an excellent safety record, they on-time performance is the worst in the world. For anything like a 5-6 hour train ride, you could theoretically fly in two hours but total travel time will be the same as the train. At least on the train you have more space.
Funny article here: http://www.thebeijinger.com/blog/201...shanghai-again