I need to know how far it is from Xian North Railway to City wall.

The North Train Station is around 16km to the City Wall. Free shuttle buses to the Terracotta Warriors Museum are available at the railway station on condition that you buy the entrance ticket of any attraction in Lintong, the museum included.

If you wanna see the warriors after the City Wall, no need to go back to the North Station. Instead, you are recommended to take a taxi to XA Railway Station, at the east square of which you can take Tourist Bus line 5 (no. 306), 914 or 915 to the museum.