Thanks for the link, it's good to see they have finally added foreign ID purchase.
For this Friday I checked availability. There are seats available but only in Chu Kuang of ithose I checked. 3.5 hours is painful for us and the kids although doable if absolutely necessary (would rather pay extra to go faster).
Probably would have to book earlier of course to have a chance at all for the express trains. But as I mentioned my wife has checked regularly before and never been able to get.
It's a pain to check availability one by one, for a company that gets billions in investments they are really far behind the times.
That said at least buying a ticket doesn't cost an arm and a leg. Interesting to note maximum you can book online per ID is six.