Booking train tickets to the Great Wall

I am going to Beijing during the May Day holiday and hope to go to the Wall. I plan to go on the Xizhimen train but am aware this weekend may be busy and would like to book tickets in advance. I have requested at the hotel I stay in but have not had a response and wonder if there are any ways I can book ahead of time to be sure I get a seat. I am a foreign student in South China but only arriving in Beijing the day before. Thanks.