Are there bus service throughout the day to get back to Xian North Train Station or Train Station?

Yes, you are right.
The free buses back to the North Railway Station are from 10:30 to 19:30 with the interval of around an hour.
In addition, the direct coaches from the scenic spot to Main Railway Station are not free. The schedules are from 08:10 to 19:00 with the frequency of 15 mins. It costs CNY 7 per person.