As a total fantasy excursion train, I double head a VL BB with a UP 9000 18 wheeler. They pull 12 Lionel aluminum "Superliner" cars from days of yore with no sweat (they are heavy cars, not particularly free-rolling and the lighting, which I have not converted to LEDs yet, eats up a lot of current).
By contrast, I put 4 of those cars behind a VL Niagara and it threw a traction tire pulling them around an O72 curve. Fortunately it did not shred and I put it back in place without having to take off the running gear. It then pulled seven MTH ABS passenger cars with no problem.
I don't run long freight consists but I do run the full sets of certain passenger trains I have. Seven or more 21" aluminum cars requires no small amount of power/traction. My Legacy AC12 has pulled 10 such cars on its own with no issue at all.
The Niagara has the "force coupler" feature which reflects the load on the engine in sound effects. That probably does not scientifically reflect the real pulling power of the engine but it does indicate that curves and any even slight grade or uneven trackwork make quite a difference to what the engine can pull. (This feature was just a bonus as far as I was concerned but having got it I think it adds no small play value.)
