The Metropolitan Washington Airports Authority on Wednesday approved a $1.6 billion contract with two engineering firms to build the first half of Metrorail to Dulles, as well as another $100 million contract to help manage the project.

In a unanimous vote, MWAA cemented its agreement with Bechtel Infrastructure and Washington Group International — together organized as Dulles Transit Partners — to design and construct the rail’s initial 11.6-mile phase from West Falls Church to Wiehle Avenue. The move follows months of closed-door negotiations that ended in late March.