What got me thinking about this subject (again) was GRJ post showing the cool Jeeps he has 3D printed. Anyhow I won't say jeeps were never moved on a flat car during WW2 as O gauge equipment generally has them. But when they were trying to get them to where they need to be, 2-4 per flatcar just really didn't make sense. Anyhow for those that are into Scale representation please look at this link.
Remember the original Jeeps were WAY smaller than the Jeep Wranglers you see driving around today. When Jeeps got Bogged down in wartime, 6 or more GI's would pick them up by the bumpers and sides, and get them back on solid ground. Total weight was about 2400 lbs "wet".