A copyright is a unique adaptation of an idea. A good example is a photo of a train signal; that photo is capable of being copyrighted.
The photo is not the idea itself. The idea of a train signal is not copyrightable.
So if you make an "exact" 1/4" scale model of an existing train signal, is that copyright-able??. Since the model is not unique being a copy, it is not copyright-able. The difference between the photo and the model is what else is around the signal in the photo. If the photo is taken at a unique orientation that is not a copy from another photo it is copyrightable.
So could the signal manufacturer claim your 1/4" model infringes? Yes, but only if their signal is unique and has been copyrighted. (Patents are whole different issue.) Which is why toy train manufacturers that make models of existing engines with unique markings must pay to use those original copyrighted designs on their scale models.
Further, if I take your 1/4" scale signal model and make my own model of your signal model, I have not infringed because you do not have or could ever have a copyright on your scale model of an existing signal. Your signal model is NOT unique.