Classic UK Minitrix Models  -  Britannia
The gearing on the Britannia (the early version) is different from that on the Mallard and the Flying Scotsman, which gives the latter a much higher top speed.   I suspect that later versions of Britannia have the same arrangement as the Mallard and Scotsman - but I have yet to verify that this is the case.
Here (top to bottom) upturned Scotsman, Mallard (no valve gear) and Britannia.  Note the size of the gears against the wheel.  The Scotsman and Britannia have gears which almost reach the end of the spoke.  The Britannia at the front has much smaller gears.  On the Britannia, you can also see the larger gears which sit in between the wheel axles.  Those on the Mallard and Scotsman are smaller and out of site.  This may mean that the holes chassis for the intermediate gears are in different positions.

This by itself does not affect the overall gearing, but the size of the gear driven by the worm gear on the motor does affect it.  Note that the Britannia has a black double cog, with a smaller gear driving the wheels.  The Mallard has a red single cog which drives the wheels directly.

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