JD, these are done on Epson heavy weight archival matte photo paper with Epson archival inks. They will outlast all of us with just a little care.
I use and recommend 2” carpet tape to apply the sides. Apply the tape to the back of the side, trim and THEN remove the backing paper. I changed to the carpet tape after trying to remove some from a floor after pulling up the carpet. It took a lot of effort to get it off this floor. After several years of making these I have never had a side release on its own.
I use photoshop to design and print the sides. I started out trying to copy cars and the clean-up time was extensive so I started making my own art work. I now have a bunch of templates I can use. I don’t worry about rivet counting-this is Marx, after all. But I strive to represent the car I’m designing. Thanks for your interest. We can discuss doing a Powhatan Arrow sometime. I have done NYC sides to match Marx NYC coaches plus some fluted stainless steel cars. I also ha a full complement of MONON, bla/gold and red/gray. I also have NKP, B&O, and a good representation of PRR passenger cars. It won’t be a great stretch to make some NW maroons.
Joe