The management of the Astor family’s Hotel Knickerbocker was baffled.

Only days after its opening on Oct. 24, 1906, the 15-story hotel at Broadway and West 42nd Street was already bursting at its Beaux-Arts seams. Two-thirds of its rooms were occupied. Diners were being turned away from its restaurants.

“Where do all the people come from?” the managers asked.

The answer was right under their noses. Actually, it was under their feet.

Among its innovative features, the Knickerbocker had been built with a direct entrance from the downtown platform of the Interborough Rapid Transit subway line. You couldn’t miss the door into the hotel, with its bold lintel proclaiming “Knickerbocker” in capital letters.