The Monroe Transfer Tunnel is a pedestrian tunnel running between State and Dearborn streets under Monroe street. It provides unrestricted transfer between Monroe on the Red Line and the Blue Line station of the same name. It was constructed between 2015 and 2016, opening on September 17th, 2016. Like the Jackson, and Washington Transfer Tunnels, it provides an unrestricted 24-hour transfer, but unlike the two, numerous original and chain retailers and restaurants have set up business there, providing a place to eat during transfer or other purposes. Unlike the tunnel, these restaurants are not all open 24/7. The tunnel opened on the same day the Washington station on the Red Line, the Racine and 58th stations on the Green Line, the Green Line extension to River Forest plus the Forest Park station on the same line, and the Main and Asbury stations on the Yellow Line all opened. 9/17/16 is sometimes known as one of the biggest days of growth for the 'L' as a whole.
For the entire history of the State Street Subway, the Monroe station was the only station with an equivalent on the Dearborn Subway but had no direct connection. To create a symmetry and convenience for passengers, the CTA decided to create a transfer tunnel between Monroe on the Red Line to Monroe on the Blue Line.
The construction included a transfer tunnel between both Monroe stations' Monroe-Adams mezzanines, which would be on the upper level, and a lower level transfer tunnel between both Monroes. The Monroe-Adams mezzanines at both stations would need to be temporarily closed for this construction.
The lower level transfer tunnel would require temporary closures of both Monroe stations as well, this time full station closures. The CTA promised that if the station project went over-budget, they'd stop construction and reopen the stations, instead of keeping the project in limbo.
On July 31st, 2015, an accidental explosion under the platform resulted in more of the tunnel being widened than intended. However, the CTA had the idea to use this space instead of repairing it. They decided to convert it into a space for retailers and restaurants, much like the Chicago Pedway. They sent out bids for who would claim the retail space in the tunnel. Some local chain and original food businesses, Subway Subs, Uno's Pizzeria, and Dunkin Donuts immediately signed up.