Chipper Jokes

This story was related to me yesterday at lunch by a fellow manager, who
heard it from his dad (guaranteed true...) Phenomenal testimony that
physics shall not be denied, with some small humor value as well.
This story involves railroad cars, Denver and a fascinating gadget used in
auto wrecking yards called a "chipper." Apparently this device is fed
old auto carcasses, and it in turn produces manageable-sized "chips" of
metal.
Seems that on this eventful evening, four gondola cars were filled by a
chipper and headed out of Denver around dusk. Somewhere along the track,
on an uphill grade, something mechanical failed on one of the cars, and
the train pulled to a siding to uncouple it. The dutiful crew chocked
the wheels with rocks, wood chunks, etc. and then proceeded to unhook the
car.
Seems no one had the slightest idea of the mass being packed in that unit,
as the rocks/wood held it in place for about 6 more...