CHINA T UNION

HI! I The infos checked on the web about china t union card are not clear.
My 15 days route is: Beijing, Datong, Xi'An, Shanghai.
I'll arrive in Pek International Airport T2. Which is the best way to get tickets metro for all my routes? I have a Wechat account with credit card linked (but my mob number will change as soon I get a chinese sim card arriving in the airport).
Is possible to buy a PHYSICAL CHINA T UNION CARD as a foreigner at the airport? If not, how I could manage it?