The SRT briefly had, perhaps for about as long as they go between derailments, the ability to purchase advance tickets on their web-site, but this went away in early 2013.

As far as I know, any advance purchase of a train ticket – these can be purchased up to 60 days in advance of travel – must be done in person, at any train station/depot (it doesn’t have to be the one you are departing from). So if you use a service, they have to send someone to a station to buy the ticket.

So ask someone in Thailand to purchase a ticket for you, or use a service, or go to a train station during the day on the 25th and buy a ticket. Your desired CoS/berth may be sold out. We’re approaching high-ish season so expect train loads to increase.

I assume that you are aware that trains/service in Thailand have deteriorated to the point of being a joke. The train to Chiang Mai might often arrive 6 hours late, assuming it doesn’t go off the rails. Note that SRT trains go so slowly that in the case of a derailment you are unlikely to get hurt, much. The trains just sort of flop on their sides.

Flying is best, safest, fastest. Government VIP, or private (Nakhon Chai Air) buses are probably safer, more comfortable and more reliable.

Thaiticketmajor does offer on-line travel ticket purchases, including air and bus, although I am not sure which payment types will work from off-shore.