I did some research on train travel but hell if I can find the sites I found helpful... it's hell getting old(right Winnie??) I will search some more and find them for you. In the mean time I did find this site that allows you to input departure and arrivals citys, date of travel. It will give you seat availability........ only downside you have to be able to read thai,
http://www.railway.co.th/seatcheck2/aseat.asp
All the best,
Magoo