D@M: I think in my case I've got it sussed.
If I complete my loco I can sell it - no questions asked. BUT, if I use engineering drawings of the same loco, purchesed from the museum that owns the loco I have to come to an agreement with regards to copyright. Which in this case could be a flat fee or a percentage. I think they'll want a flat fee up front.
Similarly, I plan to do an american loco - but if I go to the USA museum and do a photo-shoot and then sell the resultant model created from pictures they'll want to discuss a copyright deal.
However, at present, I am making the loco using reference shots from the web - freely available to all. Should I call up all those photographers and offer them a cut? I don't think so.

But then, if a painter sits in the Louvre all day and paints a copy of the Mono Lisa, does he have to pay a copyright fee?

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All the Woes of a World by Jonathan Icknield aka The Bryster

And in my final hours - I would cling rather to the tattooed hand of kindness - than the unblemished hand of hate...