Shells warp to relieve internal pressures created in the plastic casting as the item was cooling in the mold. These pressures are released gradually over time producing the warping. Most of that warping with the Alco shells happened fairly soon after the item was produced by Lionel.
In fact, they changed the mold back in the day to include pressure release areas in an attempt to stop the warping. They only had limited success with these revised molds.
If you heat the item now, all those pressures will be relieved quickly, producing even worse warping.
You can see those areas under pressure by viewing a clear plastic item with polarized light. The different color areas are different stresses. If heated, the plastic will deform to reduce those areas under stress.
Thin plastic items can be straightened with heat if done gradually and if you only wish to flatten an object or produce a simple curve. But a 3 dimensional object like a shell has too many surfaces under stress to correct, unless you could put it back in the original mold.