Restoration & Maintenance

It is always neat to see another piece of NG rolling stock restored. In some cases, almost none of the original car except may be the trucks (if it had any when recovered) and the ironwork remains. If I understand correctly, though, this is not necessarily non-prototypical because the Rio Grande & other roads sometimes did radical overhauls of their equipment periodically, and even over time a large percentage of the original "fabric" could be replaced. I guess from a regulatory and bookkeeping standpoint, if a car of #so&so goes into the car shop, and a car of the same number eventually comes out of the shop, it is the same car.......
After seeing some NG restorations and pondering this point as I was driving down I-25, I looked over at the Colo. Springs yard and noticed that not one of the standard guage RR cars looked even close to new. Bad paint, rust, ancient road names. The only new looking cars I have seen are coal hoppers. One of my questions is, how many new freight cars (non-unit) are fabbed each year. It looks like there are very few/no new boxcars / flatcars / hoppers (non-unit).
It's not a NG question, but my curiousity after seeing a string of good looking NG equipment followed by a string of ratty looking mainline SG equipment.