Middle East - How to buy train ticket from Aswan to Cairo?

I'm looking to buy a one-way Aswan-Cairo this week. I was planning on just being there for a day and now I'm reading that I need to buy the train ticket days in advance. Does anyone have any recommendations/experiences of this? I will not be in Cairo prior to this so I can't go to the station there and buy it. I am flying in directly to Aswan. Thanks.

http://www.seat61.com/Egypt.htm#How%20to%20book

https://enr.gov.eg/ticketing/public/login.jsf

Thanks. That was good information. I was actually able to reserve my ticket via email. They were quick to respond and quite accommodating at Watania. As for the trains themselves, they were quite old and run-down. Very shaky and noisy ride all the way. The food was terrible so make sure you bring your own!