Humor me on this one...

[Anonymous]

The latter reason is correct - When OO scale was created in the 1920s/1930s it was impossible to fit the motors available at the time into HO-scale models of British locos. Successive manufacturers then stuck with OO even though modern motors are smaller, presumably to allow modellers to continue to run older equipment. There have been a few attempts to offer British HO scale (Lima's early efforts to name one) but none have been all that successful. There are modellers depicting the British railway system in HO, through a mixture of kits and scratchbuilding, though there are not many of them. There are also people re-gauging OO scale models to 18mm or 18.83mm gauge (known as EM and P4 respectively) to produce 4mm scale models with the correct scale/gauge ratio. Hope this is of use or interest!

[Anonymous]

Spacemouse -
If the sound/DCC conversion of the existing Hogwarts is still in your plan:
Another useful bit I stumbled across on the Bachmann UK site while looking for other info - assembly diagrams for all their locos:
http://www.bachmann.co.uk/dloads/assmbly.php4

According to Peter (M636C), you'd look at the "Modified Hall".

Scott.