Reaching Ubon Ratchathani through Bangkok by air is the most viable option. Book a flight with Thai Airways, Air Asia, Nok Air. These carriers have two flights each to and fro Ubon daily rendering utmost convenience in both timing and cost factor.
Nearly six trains run between Ubon and Bangkok which take approximately eight to twelve hours to complete the journey and costs anywhere between 250 THB to 450 THB depending on the class of travel chose. However, these are deemed to be painfully slow and not preferred by tourists.
There are quite a few buses that go from Bangkok to Ubon Ratchathani and take nearly eight hours to reach if you have opted for the first-class carrier. However, the bus stand in Ubon Ratchathani is quite far from the core of the city. You might have to spend THB 150 or more to get there by cab or Tuk Tuk.