Differences between pulling power of Thomas trains

How come Plarail engines differ in power, speed, and functionality, even interbrand-name-ably?
It's quite surprising to me how weak and strong some engines are compared to each other.
For example, a new out-of-the-box Plarail Thomas from 2015 can pull only 2 trucks up a slope track without struggle, but a lightly used Plarail Duck pulls 18 up the same track with only very slight slippage. I haven't modded either of them at all except for paint touch-ups.

I know it probably has SOMETHING to do with gearing and motor and the drive unit, but it's still very odd to me how large the gaps are.

Part of it could simply be if the engine has been 'run in', often electric motor characteristics change as they are run and gain age, if done right the older ones can run smoother than an out of the box model.

However in this case it is likely to be down to as you quite rightly say gearing, different engine/gearbox designs favour speed and/or pulling performance. Also it could come down to the amount of weight added to the locomotives, a lot of the older ones have quite a few heavy metal weights in them do the current models have as much? This would affect speed and traction quite a lot...

[Ucwepn]

Duck runs a C battery? if so then that's why, extra tractive effort from more weight.

[HankAmericanEngine]

Howdy Miksolo!

Ironically enough, sometimes it is just up the engine themselves-- My HiT Toys Donald and Douglas are powerhouses all on their own, but sometimes want to pull a rake of only five cars and a brakevan. Others, they want to pull enough that requires the twins to double-head.

I've never seen any tractive effort with my TrackMaster Duck, even though the C-Cell battery is quite heavy compared to a AA battery-- Oliver can pull a decent rake of trucks with Toad at the end, and Duck can struggle with just three cars and a brakevan.

Sometimes it's the motor, others the weight, and even just the locomotive itself.