The floor of Pennsylvania Station’s Main Concourse was embedded Glass Vault Lights. Square glass prisms set on a steel frame embedded in Portland Cement. This allowed the light to pass through the glass ceiling and glass floor to help light the lower sub levels. This was very popular on New York City sidewalks early 1900s, to help light subway platforms below the streets.

Photo: New York, N.Y., Pennsylvania Station, concourse showing gates, indicators—Library of Congress.