I have seen some recent era Lionel flatcars that melted into the tires of the vehicle they were carrying.. Subsequent flatcars had a plastic separator to prevent from happening.
I have a MTH flatcar with Blue/White 1964 Mustangs. One set of the black tire supports for the flatcar to hold the Mustang are missing. As a result the Mustang tires sit on the flatcar surface and have started reacting with the flatcar surface. Today when I got the cars down for someone to play with I noticed one car was sort of stuck to the flatcar.
Damage to the car tires are extremely hard to see. The areas on the flatcar are slightly visible. My other flatcars which all have the black tire supports have no problems.
I put a business card under the Mustang with no tire supports.
Just a heads up.
Here is the Lionel 16955 Flatcar with load and no separator from the factory