President Donald Trump’s idea of putting solar panels on a border wall with Mexico, and using the revenue from the energy it generates to finance its construction, is as far-fetched as it is half-baked. Yet a more realistic environmental agenda — one that allows Trump and his Republican allies in Congress to remain true to conservative principles — is not hard to imagine.

Since Trump announced his decision on the Paris agreement, more than 1,000 cities, states and businesses have declared their intention to continue driving down emissions, because they correctly see it as being in their own best interests. (Michael Bloomberg, founder of Bloomberg LP, submitted their statement to the United Nations.) This presents an opening for Trump and Republicans, if they are willing to recognize what it requires: a commitment to the kinds of public transportation projects supported by businesses and cities.

In speaking about infrastructure last week, Trump promised to empower mayors and governors by forming partnerships with them and the private sector. He also promised to slash the regulatory burdens involved in getting projects approved for federal money, which can take a decade. These burdens are often the bane of state and local officials’ existence. Other nations have figured out how to build faster without harming environmental or safety standards. There is no reason America can’t, too.

Trump wants to reduce the approval process to two years, a worthy goal. Particularly on transportation projects, the economic and environmental benefits could be significant. But therein lies the problem: Trump’s executive budget could threaten about $38 billion in mass transit projects, which reduce both the emissions that warm the planet and the congestion that costs the U.S. economy about $124 billion annually.

Mass transit also spurs new private investment that generates economic activity, including new housing construction. That’s a big reason dozens of cities in red and blue states have rail projects planned or underway. Not every such project makes financial sense. But on the whole, the country needs more investment in public transportation, not less.