The MBTA is one step closer to putting new trains on the tracks of the iconic Green Line.Engineers and other MBTA staff members are now working with a mock-up of the possible new vehicle, according to spokesman Joe Pesaturo. Pesaturo's interior and exterior photos show a vehicle that maintains most of the design aspects of the trains on the Green Line today.In addition to the new trains, the MBTA is also moving forward with plans for the Green Line Expansion. Seven new stations are currently scheduled to open in Somerville and Medford during 2021.Last month, the MBTA showed off a mock-up of the cars that will eventually replace the trains on the Red and Orange lines.