I will try to provide a bit of an update on that sweating concern.
Looking at records of XM's constructed on the NP Ry in the later fifties, (beyond my modeling timeframe) there are products that can be certainly found in Railway Age advertisements if one looks. These are spray on products composed of cork granules in asphalt like substances. They were advertised for use on caboose interior roofs also, both as a sound and temperature insulator.When I get a chance I will try to give the names however I did note (while doing some research for our historical group) that these items were listed on the NP's spec sheets of material requirements per car that was supplied to the builders. The price in 1950-1960 dollars x 1000 or so cars (per order) was large enough that there had to be some justification. Doubly so for a frugal outfit like the NP.
I am not sure of the longevity of these as I had also seen other correspondence of these same materials later flaking off and contaminating the loads. However that was this company vs that companies product.
I will see if I can find the mfg names, so that the HS societies of these two cars might check if these products can be found in connection with the cars records.
Jim Dick - Roseville, MN