Here’s a fantastic (if slightly idealised) view of the then still-under-construction State Street Subway from a c.1940 postcard. This view looks northward along State Street from Adams, and shows the Monroe-Adams mezzanine. Note the direct access from the mezzanine level to the basement level of businesses on either side of State Street – now long since closed.

Text on the reverse side of the postcard reads:

Cut-away view of Chicago’s subway in the Central Business District. Shown are the main tubes; the downtown center platform, which is 3500 feet long; the two-way escalators to the mezzanines with store connections; and the State St. surface level. Features of the subway are ventilation, illumination, escalators, safety, comfort.

Source: CERA Chicago website – link no longer active