I once visited Terracotta Army. I can give you some advice. If you take train, you can choose to take the train at night and arrive in xian in the morning. And then, take the coach bus at fee of 7RMB at the east of Xian Railway Station to Terracotta Army (admission fee: less 200RMB)which takes you about 40 minutes. There are Huangqing Pool not far away from the Terracotta Army.
If you plan stay Xian for two days, you can visit some other famous attractions.
Shaanxi History Museum is free of Charge and Big Wild Goose Pagoda is about ten-minute walk from the musuem. Bell and Drum tower are very closed.
If you take the plane to Xian, you can take taxi or coach bus at the airport to Xian Railway Station