The nation’s infrastructure is crumbling a little bit less.

America’s roads, bridges, water systems and energy networks have long been in poor repair. The American Society of Civil Engineers, which releases a report every four years that evaluates the problem in a letter-grade format, awarded the nation a “D” in its last report, published in 2009.

The latest Report Card for America’s Infrastructure, released Tuesday, has an unexpected bit of qualified good news: the grade has inched up to a D+. It is the first time in the 15 years that the engineering organization has conducted its study that the grade has improved.

The report is showing progress in six areas, including bridges, rail, wastewater and drinking water. No category saw a lower grade than that given in the previous report, though the nation’s inland ports, waterways and levees received a near-flunking grade of D-. (The full report can be downloaded, along with interactive analysis of all 50 states, at www.infrastructurereportcard.org.)

Some connected trends have led to the shift, according to the engineering organization. It cited a rise in the private financing of public projects and renewed attention from state and local government to kick-start their own projects, rather than wait for Washington to send money. The jump in private investment was instrumental, for example, in the improved outlook for the nation’s rails, according to the report. That evaluation jumped to a C+ from a C-. The group also cited short-term increases in financing — a reference to President Obama’s economic stimulus package, which focused in part on “shovel-ready” projects like road and bridge repair.