On the campaign trail, Trump had spoken often about the importance of investing in infrastructure. Yet the president-elect had apparently failed to appreciate that the government would need to come up with  hundreds of billions of dollars to fund his plans. Cohn, brash and bold, wired to attack any moneymaking opportunity, pitched a fix that would put Wall Street firms at the center: Private-industry partners could help infrastructure get fixed, saving the federal government from going deeper into debt.