Which of Donald Trump’s many campaign promises would bring real benefits to the economy? Which would almost certainly win support even among people who voted against him? And which seems to have disappeared completely from the White House radar?

The answer to all three questions is Mr. Trump’s pledge to put his self-described talents as a builder to work by spending $1 trillion on restoring the country’s crumbling bridges, potholed roads, rust-bucket trains and shabby-not-chic airports. More than a month into his presidency, no such plan has emerged, and there are no signs that one is coming anytime soon.

Part of this could be attributed to the less-than-blinding speed with which Mr. Trump has assembled his administration. But evidence suggests that the plan is on hold for the foreseeable future; Republican sources told the news organization Axios last week that the White House wouldn’t unveil an infrastructure proposal until 2018. Congress, meanwhile, seems fixated on other issues — rolling back Obamacare, cutting taxes — while its leaders, the House speaker, Paul Ryan, and Mitch McConnell, the Senate majority leader — seem decidedly unenthusiastic about the idea of a huge infrastructure spending proposal. “I hope we avoid a trillion-dollar stimulus,” Mr. McConnell said in December.

It was never quite clear what Mr. Trump even meant by a $1 trillion plan. During the campaign he seemed to suggest that the government would spend that much money on fixing roads, railroads and the like. But two important supporters — Wilbur Ross, soon to be secretary of commerce, and Peter Navarro, an economics professor who now heads a trade council for the president — published a white paper in October proposing tax credits to private developers, a plan more likely to provide a windfall for projects that would be built anyway. The credits wouldn’t spur needed investment in water systems, mass transit and other infrastructure that are public utilities, not vehicles for private profit.