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