Railway ticket Tianjin/Beijing

I intend to travel by fast train from Tianjin to Beijing on 10 October 2014. What should I take and from which station in Tianjin? Can I travel with a 24 inch luggage and a 20 inch carry on bag which I normally carry on when I board the plane. In this case do I have to have my 24 inch bag checked before boarding? I intend to stay in Tianjin for 3 nights. Shall I buy my ticket on arrival in Tianjin or should I arrange to buy before arrival to make sure I can get a ticket? I intend to buy a first class ticket and if not available will buy a business class if necessary. Would appreciate very much any suggestion. Many thanks. Bill from Canada