Some of that money could go to the air district's incentive programs, which include helping homeowners replace gas-powered lawnmowers with low-cost electric ones, and helping businesses, farmers and industries replace or upgrade trucks and machinery, said Samir Sheihk, the district's director of strategies and incentives.

"Those programs are always oversubscribed -- there's always more demand than we have money for," Sheikh said.

In Spain, where high-speed trains have been running for 20 years, some experts said it can take decades for high-speed rail to make up for environmental damage from construction.

High-speed trains "might be green, [but] don't take it for granted," said Germà Bel, a professor of political economics at the University of Barcelona and a former deputy in the Spanish parliament. "Because there is a lot of environmental damage while the construction is on.

"The story does not begin the day that high-speed lines begin service: The story with the environment begins the day on which the first work began."

Disregarding the construction effects "gives the environmental effects of high-speed rail a kind of mythological value," he said.

To make up for construction impacts, a high-speed train line must attract enough people from cars and planes.

"If you have a new line with huge demand, it might be environmentally friendly -- at a huge cost," Bel said. "If you have medium use of such a line, you take about 30 years to recover the environmental damage done because of construction. If the usage is low, you actually have a very bad effect on the environment.

"The point with high-speed rail is whether you get dozens of millions of trips [per year]. It's very demanding, and it's not the case with any single line in Spain."

Rail officials in California say they'll do such a good job of offsetting pollution while the system is built, there will be nothing to "make up or pay back" by the time the trains would start carrying passengers in 2022.

"Long-term, therefore, the project will improve air quality in the Central Valley."