I love Thailand trains and they give you a retro experience and they are more interactive than a plane as you can walk around do things like take a shower or go to the restaurant car. I know of only one plane where it is possible to take a shower and that is going to cost you a few bucks for the privilege on the Airbus A380 in first class.
You can take trains to the north almost to Myanmar or you can go into Malaysia if you are heading south, or you can get off and head to the holiday resorts near Phuket or the Gulf of Thailand beach resorts on the Thai Gulf coast.
When buying your tickets you want to have a bottom bunk and you want a car that is further back in the train, the further the better as you will be listing to the horn all night if you are at the front of the train. If you can afford first class and there are two of traveling then it is worth paying a little extra as you sleep better knowing that the door is locked and you trust everyone in the compartment. In 2nd class I always sleep with half an eye open even though I have never has a problem.
I figured out with sleeping on a boat that it is more comfortable sleeping in the direction of travel beam to stern as opposed to across the travel direction. It works out the same on a train as the Rocking side to side is reduced when you are sleeping with the direction of the train. There is more force stopping and starting then side to side so the first class is not better in that regard as far as sleeping is concerned. If there is a lot of rocking you are tense all night trying to keep your balance.
Things to bring to help stretch the budget with would your own coffee or tea which ever that you prefer. I buy Nescafe in the single serve packets, they last for ever and they take no space and it could not be easier to prepare. They are also a great deal at a touch over 3 baht a cup that works out to $0.20.
As they have hot water available on most trains in Asia I also purchase a cup of soup, I always get that in a plastic rather than a paper container so I can reuse it for the couple of packages of ramen noodles that I buy as well. It is 14 baht for the cup of soup and 6 baht for the ramen about $0.20.
I always make a point of stopping for some munchies before I get to the train station to save money there as well. I am partial to orange slices maybe for you it is gummy bears. You will want something to snack on the long ride. They normally have a food steward that comes around as soon as you leave Bangkok offering a set dinner and breakfast, 150 for the dinner, and 90 for the breakfast. I always try to take a pass on that although asking others they find it to be a nice meal. I do however go to the dinning car and grab a plate of whatever and feed my computer electricity. The dinning car is usually very entertaining at night as the locals all sit around and drink cheap beer and cheaper whiskey which they will ask you to join in as well which I am sure will result in a few laughs on your way on the train in Thailand.