A good source for general information about a country or location and transportation to from and within are the Lonely Planet guide books. I would strongly suggest you get the one for Thailand, and if there are ones more specific to your points of interest, get them also.
Whether to take the day train or the night train depends much on which is more important to you, the trip or the destination. If the trip, seeing the country side then go for the day train. Maybe think day train in one direction and night train in the other.
You may already know this, but here it is, anyway. The Thai Railway is one meter track gauge. That does not seem to slow them down much. I believe their speeds are up to 120 kph, maybe a little more. My only ride on the system was from Sungai Kolok to Hat Yai and Hat Yai to Padang Besar, and that was 25 years ago. My origin/destination was Singapore, and the sole purpose of the trip was to complete riding of the entire Malaysian system and get in a piece of the Thai system and set foot in Thailand during a short stint of working in Singapore. The ride was interesting and on time. Their equipment is not much if any narrower than British standard coaches. Ride quality was at times exciting, particularly the northbound which was in an open window coach that was the last car of the train. Just remember, the limits of comfort on rails is well inside the limits of safety.