Apologies for resurrecting an old thread, however next time I'm in LOS I'll be using the railway quite a lot. It's dirt cheap too, having been on the BKK-Kanchanaburi route (Kwai Bridge) I recall a 3hr journey costing about ฿50...
This is an excellent site for rail travel in Thailand, or just about anywhere worldwide really-
http://www.seat61.com/
Give it a go ! I certainly found the Rot Fai Thai service very pleasant, and at very little cost.