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1) I don't understand what "tau tu" means. Do you mean "tàu hỏa"? If so, I believe that "tàu hỏa" and "xe lửa" have different origins. However, I don't exactly know how they're different. They seem to be the same in their meanings.
2) The two words are the same of definition. But they are slightly differrent in emotional tone. "Tang" is more formal than "bieu", to me. Most of cases, I think they are quite equivalent.