I had some recollection that the NMRA Magazine (or old Bulletin, or the ScaleRails version of the magazine) had something on those NP roofs but can find nothing
Note that Central Valley offers the radial roof from their NP stockcar kit as a separate part.
https://www.shop.cvmw.com/40ftRoundFreightCarRoof-3-1002.htm
Dave Nelson
The 1919 Car Builder's Cyclopedia is available to browse and print .pdf pages:
https://babel.hathitrust.org/cgi/pt?id=mdp.39015006057858&view=1up&seq=270
Unfortunately the only radial roof cars that I see are a Union Pacific example. Still, worth a look. I'd sure like to make a few copies of the Bessemer (Fig. 6) car in HO.
Regards, Ed
We need a brain trust including Ed to get the details of the radial-roof structure posted and Wayne to determine exactly what the springback of his styrene is to get the mandrel size that gives NP-accurate radius...
I don't know if it will be of any use for the plan to convert Accurail cars into ones with radial roofs, but there's a thread here which shows how I converted some Tyco 40' reefers into 36' truss-rod reefers with radial roofs.
I'd guess that it's not especially prototypical for reefers, although many wooden express reefers did have radial roofs.
The conversion, which is shown pretty-much step-by-step, was actually quite easy, so perhaps there'll be something in it of use to you.
Wayne
I know that Rapido has recently done a batch; I've got one, and it's very nice!
However, a member of my model railroad club would like to alter large numbers of stand-in boxcars (probably mostly Accurail, they're popular in our modular club because they're inexpensive and survive transport to shows well) to have radial roofs.
I'm seeing it as a potential project for my newly-acquired resin 3D printer.
My friend plans on perusing the NPRHA site this weekend (I'm not a member, so I can't), but I'm interested in other sources, especially magazines. I've got very complete sets of MR, RMC, Mainline Modeler, and a couple of others, but... not all of them are easily indexed. Further, I would be willing to consider buying books (and he's got some already for NP), if there are some really good sources.
If I'm not having much luck in those sources, we may pay to search at the Kalmbach Memorial Library (we're both NMRA members), but would like to start with sources we might already have.