Richard Barrow Train guide: https://www.thaitrainguide.com/
Thai Railways: https://www.railway.co.th/Home/Index
There are various network maps, but some sections of line have freight trains only. I've travelled on well over half the network, but am unlikely to ever cover all of it. For a start, there's at least one line where the only trains of the day arrive and depart in the middle of the night, despite a relatively short journey duration.
Part 1 Pattaya to Bangkok
The Thai Rail website shows two trains a day in each direction.
This is a lie, since there are only two trains per day at the weekend. On weekdays, it's one per day in each direction. Their website doesn't explain that.
The timetable on the wall in Pattaya Station does explain that (see photo) and your other option is to check the Train Tracker on the Thai Rail site. It only shows tracking information for the train that operates.
The train to Pattaya leaves at something stupid like 6:55 am, so I haven't tried it.
The train to Bangkok leaves at 14:20 and arrives at Hua Lamphong at 18:15, so I have tried this.
The train cost just 31 baht, but I paid 80 baht for the Bolt taxi to the railway station.
The train makes a pleasant change to the bus and after arriving at Hua Lamphong, it's just a short ride on the MRT to Sam Yan or Silom.
At the moment, only some trains have transferred to the appalling new station at Bang Sue (Krung Thep Aphiwat). Fortunately, the Pattaya train isn't one of them. So I recommend trying a train from Pattaya to Hua Lamphong, whilst you still can.
The daily train is non-aircon, so I guess it's not quite as appealing during seasons with high humidity. The extra weekend train has aircon.
You can travel south as far as Ban Plu Ta Luang. The remaining sections of line are for freight only.
