Excavation work began on the 1.2km tunnel last March and according to Mr Diandino Peña, director of the Office for the Reorganisation of Transport (Opret), the project involved "geotechnical work of a complexity never before seen in the Dominican Republic."
Commissioning of the four-station extension is scheduled for April or May next year.
The initial María Montez - Eduardo Brito section of the east-west line opened in April 2013.
