SAY WHAT YOU WANT ABOUT MUSSOLINI: Nearly a century after the planning started, a New York subway arrives.

To understand the enthusiasm over a subway, something that’s normally the subject of grousing, consider that New Yorkers have been promised a Second Avenue subway since 1919. Over the ensuing century, the Second Avenue subway became the punch line of jokes and a cautionary tale about the ineffectiveness of infrastructure development in America.

“When you’d mention the Second Avenue subway to someone, they would react with a Pavlovian giggle,’’ said Jeffrey Zupan, a transportation expert and senior fellow of the Regional Plan Assn., an urban policy group. “Nobody believed it would ever happen.’’

Groundbreakings have been frequent, but each time wars or recessions would intervene to jinx the project. Meanwhile, officials were so confident the line would be built that they demolished elevated rail lines on Second and Third avenues, leaving the fast-growing Upper East Side of Manhattan with only one badly overcrowded subway on Lexington Avenue, two long blocks to the west.

Don’t count your blue state infrastructure projects before they hatch.