It's a great way of getting around (and is as cheap as chips - even for 1st Class) if you have a bit of time or using the overnight services to save a bit of cash on travel/accommodation costs.
Kanchanaburi Bridge (River Kwai) to Nam Tok is only a couple of hours and gives you time to see the cemeteries and take in the sort of conditions the POWs (and local labour) had to work in. As has been mentioned, Chiang Mai is worth the overnight trip and the southern line takes in Hua Hin, Chumphon (for Koh Tao) and Surathani (for Koh Samui).
The best site by far to start reading up is seat61 ..... it also covers the other countries in the region.
Seat61 - Thailand