xian subway to railway station in the morning

May I know you refer to the main rail station or the north station (Beikezhan)? For the former, it is a little risky and complicated (transfer is needed) to take metro, because metro doesn't lead there directly and you have to walk a lot. It is best to take a taxi. It costs around CNY15. For the latter, the first train leaves from Bell Tower around 06:30 and it takes around half an hour to the north station. Ur time is genrally enough. Hurry up with check in and boarding!