Do you mean the 2004 coin showing Richard Trevithick's "Penydarren" locomotive?
If so, you should bear in mind that the small cog on the top row is only attached the the large one adjacent to it. In fact it simply drives the larger cog. That large cog is connected to the two cogs on the driving wheels. There is an error in that the small cog on the upper row should actually be adjacent to the chimney and not, as shown, at the opposite end of the locomotive. But this does not impinge on the working of the cog system.
But then, you may be talking about something else entirely!