What does "The Third Level" refer to?

"The Third Level" refers to a mysterious underground level at Grand Central Station, New York, that the protagonist Charley discovers. Grand Central has two actual levels this hidden third exists in a different time.

Charley finds himself transported to 1894 people, clothes, newspapers, currency all from the 19th century.
He tries to buy tickets to Galesburg, Illinois (a peaceful place from memory) but his modern money is refused.
His psychiatrist friend Sam disappears a letter later found from Galesburg, 1894 suggests Sam found it too.
Symbolizes escapism the human desire to flee modern stress into an idealized, simpler past.
The Third Level is a mysterious, time-warping platform that transports Charley to 1894, symbolizing the human need to escape modern anxiety.