Just a quick little video of a recent project. I took a Lionel Ready to Play but not G gauge set, narrowed up the wheels to 45mm, I bought one car super-cheap to experiment, the rest came after #1 was successful. I used a Aristo centercab brick and added a styrene/balsa/styrene-covered nosepiece for the “engine” its very much based on the early Lionel O scale streamliner from the 1930s. I like the early streamliner vibe captured with it.
FYI I did try to modify the F3, but while the front truck can be narrowed, the rear is 2-3mm too wide and will need serious surgery to narrow the plastic frame box.