The 25.8km, 21-station line, which is expected to cost $US 2.15bn, would span the city's Maryland suburbs in both Prince George's and Montgomery counties, linking New Carrolton with Bethesda and intersecting with Washington's Metrorail network in four places.

State funds will come from an increase in the state fuel tax, approved by the state government earlier this year, which will be implemented in several stages.

Earlier this year Maryland Transit Administration said the target completion date for the Purple Line is 2020, five years later than earlier projections.