On a related topic, take a look at the locomotive shown in the linked photograph. This is a locomotive that ran on the Chicago and Alton Railroad. I believe that it is an 1857 Swinburne, Smith & Co. built locomotive of the "Brandt Engine" design, with an 1857 model Griggs diamond fire arrester (aka smokestack).
[Hopefully, you'll be able to see it in detail. I have looked at it on two different computer monitors. On one, the detail shows up, but on the other it does not.]
I have looked at hundreds of photos of 19th Century locomotives and have seen nothing resembling this. It appears to me to have an "ironclad" boiler. My theory is that perhaps it was thought to be prone to attack as a locomotive carrying Confederate prisoners between Alton and Chicago.
Anybody else think that this is armor plating, or have any other idea what it may be?