The should be C trains between Beijing and Tianjin. You can find the timetable in the train serach page of this web. There are many C-trains every day and there is one train leaving every 15 mi nutes. The first C train is at 6:35am and the last train is at 23:00 from Beijing. So, you can get to Beijing South Railway Station to take the train at any time between them. The train ticket is 58RMB per person, second class seats.