The approval takes the project a step closer to securing a $US 790m FTA Capital Investment Grant (CIG), which will cover 25% of the estimated $US 3.1bn project cost, which includes contingencies and the acquisition of 20 LRVs.

The 12.2km line will run from Angle Lake in SeaTac to Kent/Des Moines, South 272nd Street and Federal Way Transit Center.

Demolition and utility diversion works are due to begin in the autumn, with major construction set to start next year. The extension will open in 2024.

The project will be funded through the Sound Transit 3 (ST3) Proposition 1 ballot measure approved by voters in the Seattle area in November 2017, which will raise $US 27.7bn for transit investment over 25 years through increased exercise, car and property taxes.

ST3 will expand the light rail network five-fold to 187km, boosting ridership to around 600,000 passengers per day.

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