0 0 0 ▁new ▁trans it ▁users . ▁ ▁While ▁on ▁the ▁drawing ▁board ▁since ▁ 1 9 9 0 , ▁the ▁project ▁has ▁proceeded ▁in ▁fits ▁and ▁starts . ▁Const ruction ▁finally ▁broke ▁ground ▁on ▁Ph ase ▁ 1 ▁of ▁the ▁Green ▁Line ▁Ext ension ▁in ▁ 2 0 1 2 , ▁with ▁service ▁target ed ▁to ▁begin ▁in ▁December ▁ 2 0 2 1 . ▁Const ruction ▁reached ▁ 2 0 % ▁completion ▁in ▁November ▁ 2 0 1 9 . ▁ ▁Route ▁ ▁The ▁Green ▁Line ▁Ext ension ▁is ▁planned ▁to ▁have ▁two ▁branches , ▁which ▁will ▁split ▁just ▁past ▁a ▁re located ▁ ▁station ▁in ▁East ▁Cambridge . ▁The ▁project ' s ▁two ▁branches ▁consist ▁of : ▁▁ ▁A ▁main ▁branch , ▁ ▁long , ▁to ▁Med ford ▁within ▁the ▁existing ▁Low ell ▁Line ▁comm uter ▁rail ▁right - of - way ▁with ▁five ▁stations ▁and ▁the ▁termin us ▁at ▁▁ ▁in ▁Med ford . ▁The ▁" Med ford ▁Branch " ▁will ▁become ▁part ▁of ▁the ▁" D " ▁Branch , ▁with ▁intermediate ▁stations ▁located ▁at ▁, ▁, ▁, ▁and ▁. ▁ ▁A ▁second ▁branch , ▁approximately ▁ ▁long , ▁to ▁ ▁in ▁Som er ville . ▁ ▁This ▁branch ▁would ▁follow ▁the ▁existing ▁F itch burg ▁Line ▁comm uter ▁rail ▁right - of - way ▁and ▁includes ▁a ▁single ▁station ▁at ▁Union ▁Square . ▁ ▁The ▁Union ▁Square ▁Branch ▁will ▁be ▁designated ▁as ▁part ▁of ▁the ▁" E " ▁Branch . ▁ ▁Additionally , ▁the ▁project ▁will ▁include