Its not always heat related. Most all is related to defects in the mold design creating stress in the original shell when it was molded. Over time these stresses cause the shell to develop the "hump back" look. The F3's are also prone to it. One has to remember that the molding of plastic for this large of a train shell was in its infancy at that time, so Lionel was learning as was everybody else. Up to that time, most things were diecast metal or Bakelite. Some shells have more hump that others, some are nearly dead flat or as it should be shaped. The heat from the motor in the smaller Alco shell might have sped up the process of the plastic "relaxing" its stress points from the molding process faster, hence more of them with the issue over the F3's AD
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