4 pistons and 2 set of drivers on a non articulate frame=on a riget frame..would be consider a Duplex
It is not a rigid frame. It is articulated, as are all Meyer type locomotives.
No wheels are rigidly affixed to the boiler; all are mounted on bogies placed directly under the boiler/cab unit (comparable with a modern diesel or electric locomotive). This compares with a Mallet, where the rear set of wheels are attached to the frame, and only the front set swivels as a bogie.
http://en.wikipedia.org/wiki/Meyer_locomotive
I think the best answer to Hot Water's question....
I'm curious as to why the subject title referred to it as a "Mallet"?
...is probably because almost no one knows what a Meyer type locomotive is! 
