The train is definitely the cheapest way to travel in Thailand. The third class tickets are very very affordable. The train average speed is quite low, about 40 km/h or less and thai trains are never on time but it's still a pleasant way to travel if you have time. There are several levels of comfort, several types of more or less rapid train (regular, fast, express or super express). The railway network in Thailand consists in four lines, the Northern line, the Southern Line, the Eastern Line and the North Eastern Line.