Thai trains are rarely on time. They usually leave on time from the line departure city and accumulate almost always a delay more or less important. Train is the less expensive transportation in Thailand. You can book your tickets in advance online (from 60 days before departure). For cabins for overnight trains, it is strongly advised to book in advance. For third class tickets is not necessary but know that you will not specific seat and depending of the passengers number you may not be sitting. But traveling in third class is an experience to leave for its atmosphere. Have a nice trip from Bangkok to Buriram !