If she is travelling alone or arriving at night,better stay one night in air port hotel, and fly next day to udon,nong khai only approx 45mins from udon. the last thing she would want is to do after a long flight is to get a train ticket and sit on a slow train to nong khai,unless she is a bit of a adventurer of course.

Thank you very much for all good advices and your offers to help her. I will give her all information and then she had to decide train or flight. I write again if she need more help.

Benny, my two penny worth. I have never taken the BKK to Udon train, but the first time that I went to Thailand I did take the train/ferry from BKK to Ko Samui. We stayed a night in BKK to recover from the Heathrow/BKK flight then took the night train with first class sleepers the next night. I was exhausted for at last two days after that, hardly slept at all. OK I am an old guy but my travelling companions were not and they were no better than me. One of them (25 years old at the time) has a real fear of flying, hence the train. HE suggested that we flew back to BKK when it came time to leave.
OK, everyone is different, Polehawk loves the train, I don't think I would try it as the flight is so quick and conveniently timed as far as ex Heathrow flights are concerned.

Ok Awede try this www.railway.co.th So much talk about it I decided to try the night train to Bangkok myself first class tickets round trip day trip on the way back. Three people 2800 baht