Last visit i went from Bkk to Surin to visit my ex-gf by train and it was really a good experience i booked the first class in a train called "sprinter" and the standard are pretty good and they serve food and you can buy drinks on the way and even order food from the staff.About if its safe when you are pissed,i would say you just have to use your common sence and as you know Thai people are friendly so i think that you will be ok.
Have a nice journey.
Jesper_dk