What route can I take from Beijing airport to Tianjin station using express train?

At the airport, take the Express Train to Dongzhimen, where transfer to local bus no. 106 to South Railway Station. Then at the train station, there are many bullet intercity trains to Tianjin.

From Dongzhimen to the South Train Station, you can also take local subway line 2 to Xuanwumen, where transfer to line 4 to South Train Station directly.

Apart from the way above, there are direct buses from the airport to Tianjin and it takes around 2.5h. In thsi way, you don't need to transfer at all.