Or do I have to go all the way to Whore Lump Pong station a few days before I depart, just to secure the tickets?
Yes I have tried the Thai Railway website and no sign of any kind of pay online service which I wouldn't want to do anyway in such a corrupt country.
If you don't believe me you can see for yourself:
STATE RAILWAY OF THAILAND
Hope you're all well and I look forward to your replies,
S