To nationalize something is to make it public, or government-run, rather than private. Many countries, including the U.K. and Germany, have acted to nationalize railroads during periods when they faced bankruptcy.
In the United States, a relatively recent decision to nationalize an industry occurred after the attacks on September 11, 2001. Before that, security at airports was run by private companies. Afterward, the Transportation Security Administration was founded as a federal agency, nationalizing the security in all airports throughout the country. The original definition of nationalize is "invest with a national character," the way you'd nationalize your house by hanging an American flag on the porch.