Are you talking about tender drive instead of loco drive, or in addition to loco drive? Part of the reason that most steam loco models don't pull much is that they're too light. In addition to plastic superstructures, manufacturers don't use lead for weights, and also add all the electronic gear that consumers seem to want nowadays. Any loco can be made to pull more simply by adding weight. If you're speaking of tender drive, Tyco made one that most people found unimpressive - the motor was in the tender and the tender trucks (or maybe only one) were powered - the loco was a dummy and was merely pushed along by the tender. One of the drawbacks usually associated with putting the motor in the tender is the effect of motor torque - because most tenders can rock side-to-side somewhat on their trucks, the motor, especially at starting, will cause the tender to lean to one side.
Tenshodo used to offer PDTs, which were powered trucks of different wheelbases - the motor was completely within the truck. Suggested uses were as replacement trucks for powering passenger cars that were used in long, heavy trains, to assist the loco - the modeller had to supply the sideframes and provide a suitable mounting set-up. They also touted them as useful for tender booster trucks - had DCC been available, it probably would have been useful in coordinating the motor speeds. I used one to re-power an Athearn RDC (single truck only), and it not only ran well at prototypical speeds, but was also powerful enough to pull a dummy RDC trailer.
I remotored a number of brass steamers for a friend, and two of his favourites were a pair of CPR D-10 Tenwheelers. Due to the small size of these locos, there was room in the boiler for only the smallest of can motors. Instead, I opted for two fairly large flat-sided can motors, mounting them on the flat inside the tenders, with a u-jointed driveshaft connecting to the worm through the empty frames of the old open-frame motors. The rest of the boiler and firebox was filled with lead. I also filled the available space in the tenders with more lead, then added slippery plastic bearing blocks to the tenders' sidesills, leaving just enough clearance between them and the tender trucks' sideframes to allow them to pivot. This eliminated the effect of the motor torque, and resulted in a surprisingly strong pair of Ten Wheelers.
Wayne
Tyco did it some thirty years ago.
Why is it today that no one offers a powered tender for a steam locomotive? In HO scale I know it would be easy. Would there be that much of a demand for one? I know they have them for European models and such, but why not U.S? On my freelanced layout my Bachmann 4-8-2 Heavy mountain can only handle about 12-15 cars when I usually like to run about 30 or so in a single train and takes about 2 F7s. But it would be nice to run the same train behind a solo steam locomotive. What do yall think about that?