Due to its design, the Mustang is a poor choice for a railroad car load - unless it's a crash victim being hauled off to recycling (Yes, they did recycle crashed planes during WWII.)
A Hellcat or Avenger would be a better choice, since the wings folded/rotated to lie alongside the fuselage.
During WWII, complete aircraft were flown from the factory to the Port of Embarcation. Naval aircraft were routinely loaded aboard escort carriers, some of which served as plane ferries. Mustangs had enough legs to fly across most ocean crossings.
Also, looking at wartime security, IF an aircraft was being moved by land transport it would almost certainly have been wrapped in tarps to hide it from inquisitive people with Germanic names...
I always get a laugh out of the Walthers catalog photo of the Bf-109 standing on its extended landing gear on a flat car, wingless and devoid of horizontal stabilizer. The Bf-109 had a notoriously fragile landing gear and would have been carried in some kind of wooden cradle. It would also have been tarped over, to keep it from attracting attention from Thunderbolts, Typhoons or Shturmoviks (choose one, depending on date and front.)
Chuck (Retired USAF QC inspector modeling Central Japan in September, 1964)