If all of the kilograms were undergoing radioactive decay, then the ones that traveled back and forth would be younger, and thus heavier, because of the twin pa

If all of the kilograms were undergoing radioactive decay, then the ones that traveled back and forth would be younger, and thus heavier, because of the twin paradox. You'd probably need a new undetectable form of radioactive decay and relativistic trains to make this plausible.