President Trump’s promise to repair crumbling roads, bridges, railways and other public works is off to a terrible start. Before his administration has even come up with the $1 trillion infrastructure proposal he has repeatedly promised, his Department of Transportation is sabotaging an important commuter rail project in California opposed by the state’s Republicans. This doesn’t send a good signal about the Trump administration’s commitment to infrastructure investment.

The Transportation Department is withholding $647 million in federal grants for a $1.9 billion project that would modernize and increase the capacity of Caltrain. About 65,000 people use the rail line every day to commute between San Francisco and San Jose — a traffic-clogged corridor that is home to some the country’s most valuable technology companies, including Apple, Facebook and Google. Caltrain plans to use the money to switch to modern electric trains, from old diesel locomotives that are prone to failure. The change will also allow the transit system to serve many more people in a growing region.

Federal officials said they were delaying the grant because they need to see if the president’s forthcoming budget includes enough money for transportation projects. That explanation is perplexing because Congress has already authorized the program that would fund Caltrain improvements. The department’s decision runs counter to Mr. Trump’s campaign promises of increased infrastructure spending.

The new secretary of transportation, Elaine Chao, could be acceding to California’s 14 Republican members of Congress, who feel electrification would benefit a high-speed rail line, being built by the state, that they consider a boondoggle. While the two projects are separate, high-speed trains are expected to use Caltrain tracks in the Bay Area. California Republicans have tried everything to disrupt the high-speed rail project, but this move is particularly damaging.