Buy train ticket to Badaling in advance

The normal Beijing train ticket is available 10 days in advance. But the one from Beijing north railway station to Badaling is special and only available in Beijng north railway station. Sorry that I'm not clear how many days in advance it will be available. However, there is no need to buy it in advance at all. Because the seats on that train are unnumbered and you my not have a seat even if buy it in advance. Just board on the train early that day.