Have travelled to Udon Thani a couple of times on the overnight train and once to Surat Thani. For all these trips I started at Pattaya catching the afternoon train to Bangkok. This train is slow and takes about 4 hours but I enjoy train travel and like looking at the scenery.
I work for the railways back home and have always enjoyed train travel. I try to get in at least one long distance trip a year. Been with my wife for 17 years now and on one of our earlier trips to the village outside Udon Thani we caught the overnight train from Bangkok. She kept on being asked at her village why she was with someone that couldn't afford to fly up there. More than happy to let the villagers think I am poor.