This describes one pretty well:
https://en.m.wikipedia.org/wiki/Garratt

The middle section of the three has the boiler and cab, and both front and back sections have the cylinders and driving wheels. The back one has the coal bunker on top and, from the picture, it looks like the front one has the water. (I would expect it must be important to get the weight distribution right to maintain traction as the coal and water reduces.)

The tricky part of this concept is the need for flexible high-pressure steam pipes between the sections!