It's based very loosely on a Kerr Stuart Sirdar 0-4-0 Tank Engine. I've changed the dimensions a little to allow the use of 42mm copper tube as the boiler barrel and fitting of PPS Steam Models osscilating cylinders. I also added some crank weights to add something else to watch in the absence of valve gear! The second image is a quick render of the 3d model of the loco which I've used to generate the toolpaths for cutting out the brass parts. The boiler in the background was a piece of 1 1/4" copper tube I used to practice my silver soldering on.