A steel beam from the World Trade Center’s fallen South Tower was displayed Monday at a Long Island education center, where hundreds of families devastated by 9/11 tearfully flocked to the mobile memorial. “We can’t forget,” said a weeping Rosemary Cain, the mother of slain Sept. 11 FDNY firefighter George Cain, to The Post. Cain was brought to tears at the sight of the 21-foot-long, 16,000-pound steel beam, which a police pipe band somberly escorted as it was hauled to the Lieutenant Michael P. Murphy Navy SEAL Museum in Suffolk County on a special truck as part of a cross-country…