We will be staying in Renaissance Wuhan and will like to take a train to Xian.

From your hotel, you can walk southeast around 820 yards to get to Yuejiazui Station. Then take subway line 4 to the Main Railway Station. After arriving, there are several direct high-speed trains that can take you to Xian North Railway Station. The train schedules are 09:22, 10:26, 11:33, 12:41, 14:35, 14:56, 15:46, 16:53 and 17:48. The train takes around 5 hours on the way.

Thank You.

We will be in WH in early March for a week. Are we able to buy the high speed tain tickets to Xian easily when we arrived?

Yes, you can buy the tickets when you arrived. Or you can also book them online in advance. As I know, TravelChinaGuide provides this online ticket booking service. If interested, their contact information is bookingtravelchinaguide.com.