Hua Hin is the Royal resort and the King used to travel down there by train. It is a great station with a library next to it housed in two old carriages... http://www.frangipani.com/wordpress/wp-content/uploads/2013/05/huahin-traincarreadingroom.jpg For reference, this site seems to have a fair amount of imagery on it, including sprinters...
http://portal.rotfaithai.com/
http://gallery.rotfaithai.com/details.php?image_id=9766&mode=search
It also looks like Rotfaithai members are producing models for train simulators , so they might have drawings... http://portal.rotfaithai.com/modules.php?name=Downloads When visiting my local model train shop (Zug Haus Fortune Town), I have seen models of Thai trains on display, but at HO size on HO track, so out of gauge. The best one I saw was a model of the King's train - I think it was going for something crazy like 100,000 Baht. The shop is always busy and I think model railways are quite popular out here. I've used Zug Haus a fair bit for buying scenery for my layouts. The owner is a really friendly guy. Here are the domains of the Thai model shops I know of... http://www.modelrailroadsiam.com http://zughaus.yolasite.com/ http://www.trainsforthais.com/index.asp
https://goo.gl/4UsKP9 (Sense of Scale at JJ Mall)
Thai made models of Thai trains... https://goo.gl/C6YxLi https://goo.gl/HB5387