I have two of the old tank engines. They have plastic boilers, but still way a ton. They (at least) used to have sagami motors in them and can out pull most other engines. The old mantua F-units would pull 30+ cars, but their mallets were much more powerful than that. I used to send mine with enormous trains up the 7% grade to my coal mine.
grewsome, you will enjoy! I recommend considering stripping the driver tires white when you repaint her, and be careful of the plastic (mine were 10+ years old when I repainted them, and the plastic was VERY brittle). You might also want to consider some brass detail parts...I think Precision Scale used to offer a detail kit for those engines. I love Mantua's mallets...they just don't fit either of my prototypes now.