Where to pick up the rail tickets at the Bejing Railway Station

I will be arriving at Beijing by air around 3 pm in the afternoon on Sept 1, 2014. I would like to pre-book the overnight train on the same day from Beijing to Datong, ie leaving Beijing at 0035 and arriving at Datong the next morning at 0662 the next day. I understand that Travel China will provide me with a voucher to pick up the tickets at the railway station but where do I go specifically to exchange the voucher for the tickets and what is the closing time for the ticket office?