On my trolleys I usually make a pattern either printed or scratch built and cast a bunch in resin, I'm talking mostly seats. I've even made milk glass ceiling lights.
I use micro plywood stained and varnished mahogany for interior walls. This is an opportunity to let your imagination run away. These old passenger cars were works of art.
One reason I like N scale is I can mostly ignore the interiors.
You are going to love O scale !
