Buying train ticket at another city

Itinerary: Next month, I plan to travel Yinchuan-Yan'an by bus and connect Yan'an-Xian North by HSR train on the same day.

Problem: I do not know the Yinchuan- Yan'an bus departure schedule until I arrive at Yinchuan South Gate Bus Station. Only then, I can buy connecting Yan'an-Xian North HSR train ticket for the same day journey.

Question: Can I buy "D" class HSR train ticket at (1) a train ticket agent in Yinchuan or (2) Yinchuan Train Station, for Yan'an-Xian North HSR seated train ticket?