Frank, Matt may be right, but I think you can probably get a ballpark
answer this way: very few cars in his series were rebuilt in 1955;
nearly all were rebuilt by 1970. If you assume there was a roughly
linear shop rotation of these cars during that interval, you get 7
percent per year rebuilt, or a crude guess on the order of 35 percent
with rebuilt side sills by 1960. Judging from looking at a lot of
photos, I think this isn't far from the mark.

Matt Herson might have more specific data, I know he has looked into
this. A significant proportion of cars in this series were rebuilt
before the NP's adoption of the post-steam-era billboard/jumbo monad
paint scheme.

It's beyond this group's era, but relevant to your quetion that in
years of looking I have located only one or two photographs of NP 40ft
AAR steel boxcars retaining their original tab sills by 1969-70, out
of hundreds of cars examined.

Chris Frissell
Polson, MT

In STMFC@..., "Matt Sugerman" <camas74@...> wrote: