A lot really depends on the exact day/date(s) of travel as availability may be limited.

The train does save on one-night's hotel stay so factor that in to any cost equation.

Some tracks have been repaired, certainly the spots were most derailments occur, but the tracks still get flooded.

The new Chinese-manufactured carriages were due to go into service two days ago, on a few select train route numbers; I think the fares on these carriages are much more expensive.

A LCC flight from CNX to BKK with a 3 hour connection should be fine, I might make that 4 or 5 hours for an LCC flight to DMK, allowing for a ground connection between DMK and BKK.

You could always exit or board the train at a point north of the city, like Bang Sue, which might save an hour or two, utilizing a taxi from/to BKK.

On-line booking remains problematic and challenging; you can purchase tickets 60 days in advance of travel in person at any train station, or 30 days in advance on-line.