Fallacious yet widespread and documented beliefs courtesy of Wikipedia.
Toilet waste is never intentionally dumped overboard from an aircraft. All waste is collected in tanks which are emptied on the ground by special toilet waste vehicles. A vacuum is used to allow the toilet to be flushed with less water and because plumbing cannot rely on gravity alone in an aircraft in motion. The infamous blue ice is caused by accidental leakages from the waste tank. Passenger trains, on the other hand, have historically flushed onto the tracks; however, modern trains usually have retention tanks on board the train.