Well for one thing, differing number of gear teeth from the motor gear and to the gears off that. And probably slightly differing specs on the motor itself. Both Lionel and K-Line use a very similar looking Mabuchi motor, but the K-Line ones run faster at lower voltages than do the Lionel ones. All my Lionel engines of this sort (save for the Industrial Switcher) all need the A-U voltage setting on my 1033, whereas the K-Line engines run so much better and slowly with the B-U voltage setting.
Also I have found the motors don't always run at the same speed, even on a dual motored engine. And it's not just the low-cost truck mounted motor engines. I've heard the same thing about LionChief Plus engines, which have a bigger and more costly vertical motor with flywheel.
I've had a couple of dual motored engines over the years where the motors clearly to the eye were running at different speeds. So I just swapped motor trucks and got them to where the speed was closer. And the engines did run better after that... meaning less gear growl.
Back in the day when you could dial an extension at Lionel, I was friendly with one of the repair technicians. I was curious as to how much weight could be added to a basic starter type single motored diesel. My guess was that since Lionel recommends a 10 car limit for the 4-4-2 die cast starter steamer (it will obviously pull more), weight equaling the die cast shell weight could safely be added. And that's what he said to me.
So I've been experimenting with single motored engines. Since my current layout is smaller, 10 car trains are my limit. And I've found with some added weight over the motor end, with one or two traction well-adhered traction tires, I have no trouble what-so-ever pulling a 10 car train, with very minimal slow down on the curves.
