Hi!
We've compared prices of train tickets of Dticket, Baolau and 12Go.
Dticket is the official booking platform from SRT. It makes you register and it's not super user-friendly.
Baolau is a private company offering e-tickets adding a small fee for ticket.
12GO is another private booking platform that offers paper tickets (also some e-tickets in some cases) also adding a fee.
Here you have an example of the prices of the train from Bangkok to Chiang Mai:
Here you have a link in case you want to know more about the Thailand train ticket price comparision.