Description from American Experience:
In 1910, the Pennsylvania Railroad successfully accomplished the enormous engineering feat of building tunnels under New York City's Hudson and East Rivers, connecting the railroad to New York and eventually to New England, knitting together the entire eastern half of the United States. The tunnels terminated in what was one of the greatest architectural achievements of its time, Pennsylvania Station. Inspired by Paris' Gare D'Orsay and the Roman baths of Caracalla, Pennsylvania Station covered nearly eight acres, extended two city blocks, and housed one of the largest public spaces in the world; many of the 100,000 attendees of Penn Station's grand opening proclaimed it to be one of the wonders of the world.
But just 53 years after the station’s opening, what was supposed to last forever, to herald and represent the American Empire, was slated to be destroyed. The financially-strapped Pennsylvania Railroad announced it had sold the air rights above Penn Station, and would tear down what had once been the company's crowning jewel to build Madison Square Garden, a high rise office building and sports complex.
On the rainy morning of October 28, 1963, the demolition began; it took three years to dismantle the monumental station. The monumental edifice was gone, but the tunnels and the trains remain, serving millions of people every year.