A really good web site and travel consultant is mattana@travelconnecxions.com They can make the reservations for you in advance, have the tickets delivered for a small fee to you hotel and give you all the information that you may need about the trains, including schedules. If I were to travel by train I would definitely go at night and get a sleeper. You save on hotel rates and at the same time get to your destination.
However, for a few $$ more, and to save lots of time, I would just fly and get it over with. Unless I were travelling to Chiang Mai where you can get some great scenery into the trip near Chiang Mai, I wouldn't bother with the train.
PEACE
David