I have 5 of the Hogwart's Albert Hall engines early and the later Lion Chief. Of all my engines all of this particular design has been a real struggle to keep them running. I use them for some shows train operations, have done different paint on some.
I have been having problems with all of these with the gears either binding or slipping. I take them apart clean and grease and adjust so not to tight; but tight enough not to slip. Within a few hours all of them are giving me problems. Here is a photo of what it looks like now slipping after only about 8 hours running time!
Worn gears and plenty of metal shavings in the gear box.
How are your Hogwarts engines running???