Bangkok-Surat Thani is the Southern Line, but according to the [official timetable](https://ttsview.railway.co.th/SRT_Schedule2022.php?ln=th&line=4&trip=1) Train #40 runs *from* Surat Thani *to* Bangkok.  Perhaps this is why you're not seeing it?

In gener, you can book Thai train tickets at the official [D Ticket](https://www.dticket.railway.co.th/DTicketPublicWeb/booking/booking) booking site, which is fortunately much more responsive than the main SRT site.

Also beware that Thai train schedules are currently in flux due to the repeatedly postponed relocation of the main Bangkok terminus from Hualamphong to Bang Sue, so check routes carefully. "Bangkok" in schedules refers to the old one.