Walk to the bus taking area B after getting out of the new baiyun airport of Guangzhou and find Kong Gang Kuai Xian Line 1. The bus will sent you to the ticket office of CAAC in Guangzhou city proper. The railway station is just not far ways from the ticket office. You can also take other aiport buses to downtown area and change to other public buses to the railway station. K648, T264, K82, K226 are trains dpearting from Guangzhou to Xian. T264 is the fatest which will run over 20 hours.