Ed and Mike, thank you for your kind comments.
The panels were cut from a foil tape (aluminum, with a backing paper which protects the adhesive until it's applied) used by insulators for wrapping industrial pipes with insulation.
There's a version of it available at home improvement and hardware stores, but mine is the industrial stuff, and, I've been told, rather expensive. I used a new blade in my utility knife to cut the individual panels from the roll, then, with the backing paper partially removed from one edge, pressed it into place, rolling the remainder of the backing off as I smoothed down the foil. The key to getting good adhesion is to burnish the surface rapidly, in order to generate some heat - I'd guess in some industrial applications the heat would already be in the pipe. If the sheet isn't aligned properly, you can, sometimes with difficulty, remove the panel, but it won't be re-useable, as it usually tears due to the strength of the adhesive.
With the backing paper removed, the aluminum is .003" thick, or roughly 1/4" in HO scale.
The Seaboard car was my first experiment in using it as full side-sheets on a model, and didn't turn out quite as well as I had hoped (due to my mistakes rather than any fault with the materials), but it's definitely a useable material for modelling.
Some years ago, I used the version from a local hardware store to replicate a real car in a photo. The model is a modified Train Miniature car, with dry transfer lettering from C-D-S. On these early steel cars, like the Pennsy's X-29s, the side sheets tended to rust out near their attachment to the sidesills, with many being repaired with sheet metal. The prototype had the full treatment to the right of the door, but only on the outer four panels to the left of the door...
The patches are still in place, so the cheaper stuff seems to stick pretty-well, too. Red Caboose offered several versions of the X-29, including one with simulated side patches.
I think that the foil is probably too thin to use as car sides without some sort of backing, but it's pretty easy to do those bulging gondola sides...
The car is from the bargain table of my former hobbyshop, a Proto model somewhat botched in assembly by its original owner. I repaired it as necessary, then stripped off the original lettering, repainted and re-lettered it, then added the signs of abuse which most gondolas seem to acquire.
Years ago, I tried this with a soldering iron, pushing the hot tip against the interior of the sides....naturally, it didn't take long before "Something stinks!!" echoed down the basement stairs. The results were a bit hit-or-miss.
The example above also used a soldering iron, but it was used only to heat the plastic, without contact. I then used the round handle of some tool to deform the side panels...
As I recall, the soldering iron was a 200 watt model - no tedious waiting for the plastic to soften. 
Wayne