OK I finally got an OTP, no idea why I did not before. There seems to be 9 days with multiple time slots available but only until 11-Jul so its only working for one month ahead and no further.
Without knowing how this works now they've changed it, when you say only 5 days left you mean only 5 days left in June and now full or over what period is full? When should one check availability again, from 1-July? How does this work?
Certainly not the one in the op. That's what is used further north from Laos to China and is very misleading in the op. In Thailand it will be the older diesel locos but they will go all the way into Laos whereas now they have stopped on the Thai side from Bangkok.
You'll be able to feel smug about contributing to building a better infrastructure for the country, less the amounts siphoned off in brown envelopes of course.