The question of how far to go in a restoration/preservation project must be answered before anything is done to the artifact. In fact the two words do not mean the same thing and different groups, museums and individuals may have a variety of established philosophical attitudes that are followed as guidelines. There are the those who take the do nothing and let nature take its course attitude, there is the stableize and prevent further decay of the item school of thought, some think the best thing to do is to save what you can and copy what is unusable and re assemble and there are those who will rebuild, copying the artifact piece by piece and make it operational. Certainly each has merit and all need to be flexible as progress on the project may uncover unanticipated issues that require some rethinking and redirection.
Here is a photo of C&S 1008 when we acquired her. Our original intent was to add a few new pieces and repair this and that, put wheels under it, paint it and run it. When we started it didn't take long to realize that the frame was so unstable that it would most likely not support the body of the car as work continued. We were also pretty sure it would not support the heavier parts of the running gear, brakes or couplers. Based on this it was determined that a new "foundation (frame)" would be required. The original was disassembled and each part measured and a drawing made. Then each part was made out of the same material as the original (oak and fir) and it was reassembled using the same construction methods as the original, mortice and tenion with horizontal and longitudinal tension rods (no nails or screws). We then assembled the original frame and sent it to Como where the 1008 actually ran, it can be found on display in the roundhouse today for visitors to view early railroad wooden car construction.
Here is the 1008 frame in Como
Richard Farmer