Originally Posted by: AshleyH 
I believe the Märklin S10 is actually a Trix model.
It is interesting that you had a broken gear,
because exactly the same thing has happened to my friend's Märklin Kaiser Wilhelm S10.
I am trying to source a new gear for him (they are plastic :( ), but we have been told by a dealer that the current S10 gears have been modified and are now thicker.
Can anyone confirm this, and if so, can anyone suggest how I can repair this loco?
Many Thanks
Ashley
Just picking up this thread...
I had one of my model RR colleagues from Belgium with the exact same problem.
I have helped him by manufacturing the darn part...in brass this time!
Theoretically, it is just a pair of coumpound gears Mod 0.3 (Märklin usually uses Mod 0.4) , which I assembled/pressed together from pre-cut gears.
It is not an easy task, as the dimensions are very small and the part are difficult to mount on the lathe and milling machine.
(How do you hold a 5 mm diam gear in a lathe's mandrel without damaging the teeth?)
In addition the margin for error is about 1/100th of a mm, otherwise the gears "bend" when re-installed in the loco.
I ended up manufacturing a couple of specific mandrels to be able to hold the darn thing on the machines.
For some strange reasons, the dimensions (primitive diameter, outside diameter) of the original plastic part *does not* match the Mod 0.3 specification, and are slightly smaller.
I have a hard time understanding Märklin/TRIX reasons for this discrepancy...so I suspect this was a CAD product, generated by an "unexperienced" design engineer (lots of theory, just no practice..)
To compensate all this , I ended up re-manufacturing the parts, replacing the 14 teeth pinion by a 13 teeth one.
Worked like a charm, providing a somewhat higher demultiplication , which serves the running characteristics well.
The only problem: the pre-cut gears + the other materials cost about €60.00...and I am not counting my time.
So let me know if you are interested...
Cheers