Recognizing the cost and difficulty of implementing an all-rail solution, the Chicago Central Area Plan was introduced. It proposes a mix of rail and bus improvements, the centerpiece of which was the West Loop Transportation Center. The top level would be a pedestrian mezzanine, buses would operate in the second level, rapid transit trains in the third level, and commuter/high-speed intercity trains in the bottom level. The rapid transit level would connect to the existing Blue Line subway at its north and south ends, making possible the "Blue Line loop", envisioned as an underground counterpart to the Loop elevated. Alternatively, this level might be occupied by the Clinton Street subway. Among other advantages, the West Loop Transportation Center would provide a direct link between the "L" and the city's two busiest commuter rail terminals, Ogilvie Transportation Center and Union Station. The plan also proposed transitways along Carroll Avenue (a former rail right-of-way north of the main branch of the Chicago River) and under Monroe Street in the Loop, which earlier transit schemes had proposed as rail routes. The Carroll Avenue route would provide faster bus service between the commuter stations and the rapidly redeveloping Near North Side, with possible rail service later. These new busways would tie into the bus level of the West Loop Transportation Center.