Steam Mallet Rotary Snow Plow

4 pistons and 2 set of drivers on a non articulate frame=on a riget frame..would be consider a Duplex..plus the duplex where first developed in Europe in the first place..

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! 

I enjoyed the great whatchmacallit debate over this contraption and if names meant a lot to me I do like Swiss Miss..throw in some marshmallows and hunker down on a snowbank to watch it chug along and the world is a perfect place.