Colin,
As one who dabbled in ship modelling for a while, I would completely agree with the sentiments expressed above. Even where you may have a good or even original plan of an 'as built' vessel, by the time it has seen service for any length of time, the chances of it being modified start to rise. If you are talking about a class of vessels, this can apply, as can the further complication that ammendments may be made, even as further units are being built, after operational experience, new, modified equipment added etc. If we are talking about bits of railway equipment, again built over time, modified, batch built by different contractors - all this occurs before we even get to ongoing maintenance, repair, upgrading etc etc. Moral: photographic evidence is every bit as necessary and potentially even more important when building good, or in your case, great models.
Best wishes,
Alastair M