Los Angeles, California - Proposed high-speed rail lines in the western United States moved a step closer to reality Tuesday with the announcement of $6 billion in government grants.

The cash will come from President Joe Biden's signature infrastructure bill aimed at revamping the country's creaking railways, roads, and bridges, and is part of a plan to double the number of people taking the train by 2040.

Half of the cash will go towards the construction of a high-speed line spanning 218 miles between Las Vegas and downtown Los Angeles, slashing the current five-hour car journey time to two-hours and 40 minutes.

The rest will be used to support a long-touted line linking Los Angeles with San Francisco.

While high-speed rail is the norm in much of Europe and Asia, particularly in China and Japan, the United States has long lagged behind.

But there has been a renewed emphasis on projects like this as the United States hurries to overhaul its polluting transport sector and make it more environmentally friendly and give them an alternative to gas-guzzling cars.

"California is delivering on the first 220-(mile per hour), electric high-speed rail project in the nation," said California Governor Gavin Newsom.

"This show of support from the Biden-(Kamala) Harris Administration is a vote of confidence in today’s vision and comes at a critical turning point, providing the project new momentum."

A line linking Las Vegas with Los Angeles, America's second biggest city, has been mooted for almost two decades, during which time its proposed cost has ballooned.

The $3-billion grant announced Tuesday represents a quarter of the price tag of the project to be built by Florida-based Brightline West, which press reports suggest could be up and running by 2028.

Nevada Senator Jacky Rosen said the line could reduce the number of cars making the journey by up to three million a year, slashing planet-warming carbon dioxide emissions by 400,000 tons annually.

"This historic high-speed rail project will be a game changer for Nevada's tourism economy and transportation," she said. "It'll bring more visitors to our state, reduce traffic on the I-15 (highway), create thousands of good paying jobs, and decrease carbon emissions, all while relying on local union labor."

Wes Edens, chairman of Brightline, said the company was honored that the government had entrusted it with the project.

"This is a historic moment that will serve as a foundation for a new industry, and a remarkable project that will serve as the blueprint for how we can repeat this model throughout the country," he said.

"We're ready to get to work to bring our vision of American-made, American-built, world-class, state-of-the-art high speed train travel to America."

Biden is expected formally to announce the grant when he visits Las Vegas this week.

Last month the White House said there would be $16.4 billion in new funding for 25 passenger rail projects in the country's northeast.