Requirements for regulatory impact analysis in the United States vary greatly. Executive branch regulatory agencies in the United States are required to conduct regulatory impact analysis before issuing regulations, and statutes sometimes require agencies to conduct some economic analysis. But when Congress considers legislation that mandates prescriptive regulations, legislators are under no obligation to conduct or even consider impact analysis. This paper documents the diverse degrees of discretionary authority Congress grants US executive branch agencies. It then presents a case study that systematically compares the quality of impact analysis that informed legislative and regulatory decisions on positive train control, a technology mandated by statute in 2008. The legislation was adopted with virtually no consideration of impact analysis. Given that major regulations are often required by statute, we conclude that regulatory legislation should be subject to the same kinds of impact analysis that US presidents have required regulatory agencies to conduct for almost four decades. Congress could develop this capability by creating a regulatory analysis division of the Congressional Budget Office (CBO) to evaluate the social benefits and costs of bills, similar to the way CBO scores the federal budgetary consequences of bills.