Purchased tickets for my wife for a unplanned trip to Bangkok and back earlier this evening . She went into sticker shock when I checked for flights down.

Link to site: https://www.thairailwayticket.com/eTSRT/Default.aspx?language=1

Found it easy to do after signing up for it (top right on page). Also good for checking schedules first if you don't want to register. 

When searching for trains be sure you select the region you are going to (defaults to northern).

It does not have a round trip option so you have to reserve and purchase tickets separately.

I paid for them with my Bangkok Bank debit card. That part was also easy with the normal verification process by getting a OTP.

As soon as you complete the purchase you get an email with the ticket attached. There is an option to print them form the confirmation page but I chose to print the attachments instead. You can also sign in and click ticket history and print them out again.

 

Very nice!  Seems easy to use. 

That new "red" train is very good. There is one every route, NE, N and South.