Bart says the decision to exercise the option will save around $US 128m because it includes an agreement with Bombardier to accelerate delivery by 21 months and provide five vehicles free-of-charge. The new trains will completely replace Bart's existing fleet of 669 vehicles.

The first of the new trains from the initial order, which was placed in June 2012, is due to arrive within two years, with the remainder scheduled for delivery between early 2017 and 2021. The vehicles will be assembled at Bombardier's plant in Plattsburgh, New York, with at least 60% localised content.

"Replacing our existing fleet is essential to BART's commitment to be a world class, sustainable public transportation system," said Bart President Mr Tom Radulovich. "The cars we've approved today will better serve our riders and help accommodate growing ridership as the Bay Area population expands and we extend into Silicon Valley."