It is not possible to buy Thailand train tickets online. Usually it is not a problem to go to the station and buy the tickets but for important public holidays it is advised to buy it in advance. The tickets can be purchased 2 months in advance only if you can go to the train station.
As we will be in Chiang Mai for Songkran (Thailand’s traditional New Year’s Day from 13 to 15 April) and going back to Bangkok from there, I was looking for a way to buy the tickets somehow from Spain.
I read in a page that asia-discovery.com can buy the tickets for you and deliver to your hotel in Bangkok for some fee. I checked some other web sites and seemed trustworthy and I decided to go with them.
When I filled the form with my details and travel date, I received an email from Royal Exclusive Travel not from asia-discovery. It made me a bit uncomfortable but I really wanted buy those train tickets. :) Luckily, from the beginning, they were very professional so I could trust them. They managed to find us a train ticket for the date I wanted and when we got to our hotel in Bangkok, it was waiting for us. Credit card payment is not charged until the tickets are successfully purchased and issued.
They even sent an email to tell me who took our tickets from the reception. At the end, I am very happy with their service and excited to take a train to sleep over the night from Chiang Mai to Bangkok :)