I'd be willing to bet Jack that if the motor on your Berk is a side mounted DC can motor (you'll see the gold spur gear off the motor armature hitting the other gears when you turn the loco upside down), which I would suspect is the same identical motor being used in most of the basic Lionel lower end Lionel locos.
Mabuchi is the company that makes these DC can motors and they are used by Lionel, K-Line and probably others. The new locomotives that have vertically mounted DC can motors have a bigger motor: there's more space for it with the vertical mount and a sensor flywheel is needed for the locos that are equipped with the speed control function.
I talked to a Lionel tech years ago that I knew and he told me the motor being used in the Industrial Switcher loco was the same one used in the GP's, NW-2's and the basic DC motored die-cast steamers. I was curious about adding additional weight to the Industrial Switcher locos, which I did, improved their operation tremendously and have had no problems. I did remove the traction tire entirely from the Industrial Switcher to help with electrical contact and prevent stalling.
5 to 7 cars is I think a conservative number for your locomotive's pulling capacity. You should have no problem with pulling 10 cars. I'm sure your dealer is being conservative as they don't want to see you "abuse" the engine.... ever notice all the normal disclaimers companies print with their instructions nowadays? Companies are getting sensitive on the liability aspect of things because of our friend, the lawyer and his friend, the lawsuit.
Although there are other variables: do you have track grades up and down, are you pulling heavier die-cast cars or cars or cars that are illuminated and have roller pickup assemblies (there is some drag from the wheel contacts on the roller pickup assembly).
My Industrial Switcher will pull more than 20 cars, but that's not something I would recommend doing on anything more than an experiemental basic. With the nature of the DC can motor, the harder it works, the more it heats up. The more frequently it heats up, the shorter it's life becomes. DC cans motors have advanatages, but when they quit working, they can't be fixed - they have to be replaced. Even my NYC Flyer type Lionel Columbia 4-4-2 steamer will pull 10 cars with no trouble. So your engine should do at least the same. Even my weighted cheapie all plastic DC motored Lionel Docksiders and 8902 pull 10 cars with ease.
Matter of fact, my nephew's red all plastic Lionel Rock Island Docksider pulls like a real champion - after I made some alterations. 15 cars and more with that little engine no sweat, but again I advise my nephew to be more careful and keep it 10 cars or less. Can't help myself... I paid a whopping 95 cents for that engine plus a few more dollars in renovations and I don't want to be out on my big "investment." [:D] Actually the big payoff is seeing how much my nephew loves that engine.
Why there's an ad in the train magazines that show that little guy is worth $350.00 [?][:D]