train tickets

Once I buy it online, do I have that reserved seat on that particular ride and on that particular day?(Assuming I pick it up at station or have it delivered to my hotel) I've been told that they can only be purchased at the station and it is first come , first served which made me think that the online ticket purchase may only give me a ticket but no real reservation for a particular ride, time. or date. Also, can I have a ticket sent to a Beijing Hotel even though I will be taking a ride FROM Xian one -way to Beijing.