I had a look around for you and found this out:

All trains going from Bangkok to Ubon Ratchathani stop at Pak Chong. Departures at Bangkok’s Hua Lamphong Station more than 5 times daily trip. Fare (excluding surcharges for rapid or express trains and aircon) is 36 Baht for 3rd class, 74 Baht for 2nd class and 146 Baht for 1st class. The rail distance is 180km. The journey maybe takes 3.5 hours.

You also can take bus from Bangkok to Pak Chong, which leaves from the Northern Bus Terminal(khampengpet rd). An air-conditioned bus is usually around 100 baht and takes two and a half to three and a half hours, to Pak Chong. Buses are suppose to run every 30 min.

I'd be inclined to take the train, hope this helps.

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