The best way is to take the ovenight soft sleeper train between Beijing and Xian. I guess your destination is the Terracotta warriors, right?

Yes, I would like to see the Terracotta warriors. I am debating whether it is worth to fly into Shamghai, take the train to Xian and onto Beijing? Or just fly into Beijing take the nite train to Xian for 1 day and then back in the same evening to Beijing. I only have 8-9 days.

Is Shanghai also one of your destinations or not? If not, you needn't fly to Shanghai to connecting the train to Xian. Just fly into Beijing to take the overnight train to Xian for 1 day. BTW, what are your exact destinations aparting from Beijing and Xian?