If that were the case then the wheel carriages would still be intact.
No, somebody either bought or found the boxcar and moved it to the site and put it on a foundation.
Hard to imagine someone not noticing over time what their house was built from.
People make living spaces out of containers every day. They are nothing but a modern version of a box car.
I lived in Bakersfield. Merle Haggards home in the Dale started as a boxcar. I also know of another home on a farm where it’s obvious one of the bedrooms is a boxcar. Probably started as the boxcar and then built around it. It’s not that unusual.