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I can only reiterate Kevin's points. It is a well documented problem. Not all sets suffer, but the majority do. You may be one of the lucky ones, but I still recommend the removal of the foam. Just store it separate from the cars so that the set remains complete. I have seen sets where the cars have been put in thin plastic bags to form a barrier to the foam.
Tissue paper is the best.Wrap them up and put them back in the box. Cover with enough tissue paper to stop them from moving but don't squash them. They will be fine. Like YYS4BOB says, keep the foam in a bag to go with the set.