Someone mentioned the Class 158 Sprinters that went to Thailand, and I have just taken delivery of some 3D printed body shells of those from Shapeways. Back in the 70s some Hunslet shunters were sold to the SRT and although they have now all been withdrawn, one is on display in the "Railway Hall of Fame" in Chatuchak Park, Bangkok
Before that though there wasn't a great deal of success for British locomotive builders selling their wares to the Thais. You have to go right back to 1920 and a large class of 4-6-0s to a design created for the Indian metre gauge lines. That sale was facilitated by WW1, and Siam (as it was then) joining the Allied side, wiping out the competition from Germany. A few years ago one was gently mouldering at the side of Makkasan works
Just a few yards further along the same siding was this curious 0-6-0T
Curious because it was built by Brush Electric Engineering, a rather unusual source for steam locos.