: : I don't think there are many common components anymore, other than : couplers, motors and gears.
What about wheels [1], coupling rods, connecting rods, tenders - if such items were common between various prototypes then something must be very wrong if they are/can not be common across models (There were even shared boilers across different classes, whilst diesels and electrics sometimes share basic bogies, but admittedly, these might throw up moulding issues between different models).
In some ways the manufactures have caused their own worst problems, since they (seemingly) turned everyone in to box openers rather than modellers...
[1] as anyone who has studied Mike Sharman's (sp?) definitive work on the subject will know