A bit of internet research told me this is GN X220 built 5/1942.

I originally thought it was one of those caboose look-alike kits they're now selling, but a quick glance at the underbody indicated that this was a real caboose.

Anyone know what era the GN used aluminum barn siding on their cabooses?

Not being a NP historian, I can't say if they ever did use aluminum siding. I'm not aware of it being used on railcars like that. That said, it looks like a good idea if you're dealing with a wooden car and are more interested in protecting the car from weather. It might also be a reasonable means of protecting a wood car that you don't have immediate ability to restore or place in protected storage. The wooden caboose that TVRM has (NC&St.L. No. 41) seems to require attention every few years.

Based on the design of the car, I'd be willing to bet that it started life as a wooden caboose. Replacing siding can be done by anyone with basic woodworking skills. That said, it costs money and is time consuming for most small organizations. Whomever put the siding on this caboose at least took the time to install it so that it looks good (mimicking the orientation of wood siding). The windows also look like aftermarket aluminum frame replacements-no way to open them.

Alan, sorry, the question was meant as humor. The siding and windows were obviously fairly new, and added long after this hack's railroad career ended. They were done with care, and seem to be protecting it against the harsh Montana winters pretty well.

Since it was intended mostly for fun, I posted this in the rail fanning topic, but that's not always obvious.