The origins of the subway lie in the Baltimore Area Mass Transportation Plan published in 1965, which envisioned six rapid transit lines radiating out from a central city loop. Planning studies from 1968 proposed a rail transit system 71 mi long.

As the vision was translated into reality, the original concept was trimmed to a 28 mi system in the Phase 1 plan, published in 1971. This plan involved two of the original six lines: a northwest line from Downtown Baltimore to Owings Mills and a south line to Glen Burnie and the airport. Phase