There is one super helpful website on that topic which is: http://www.seat61.com/Thailand.htm
All train lines, timetables, pictures, everything that you need to know.
The State Railways of Thailand own website doesn’t allow reservations.
So you have to use a tavel agency. But as soon as you read about making a reservation online the nightmare starts.
Bookings open 60 days before departure, but email bookings are only accepted more than 15 days before departure to give them time to respond.
Ok. Not very convient.
Some people said they waited 2 weeks for a reply, another correspondent says it took Thai railways 18 days to reply, and one correspondent says he waited a month, so you have to be patient.
Waiting two weeks for a reply if I want to take a train in 15 days? Come on.
You have to follow up your online booking by emailing them repeatedly if you don’t get a reasonably prompt reply, as when they are chased they usually find your booking and confirm it.
And then you look at the sites of the agencies …
Uh, „Last updated 1 February 2010“. Trustworthy. The design looks even older.
You can only book tickets from where the train starts but not from somewhere in the middle. Contact form fails with white page. The reservation form is very, very long. And why do I have to calculate the ticket price?!
Has also an antique site design and features very long reservation forms too. Would be so nice to just check if there are tickets left for a certain trip. But it’s great that I can make a booking for more than 10 people.
The best one is the following one. For every request you get:
Due to a number of rail accidents we are currently reviewing our Online ticket sales
We apologise that we are unable to assist in obtainting Thai train tickets at present
and will make an announcement should we start again.
Oh well. Guess we’ll take the bus then.