Model Power’s metal freight cars are re-releases of the old Mantua line. The flat car may be okay, but their others are much too heavy to be usable on an operational layout if you want to pull more than one or two cars, especially if you have any inclines.
My impression of them is that they are intended more for static display than for operation. The one box car I have handled had out-of-round wheels, and it didn’t roll well even after the wheels were replaced.