The U.S. Supreme Court case Train v. Natural Resources Defense Council (1974) revolved around the interpretation of the Clean Air Act, specifically whether it allowed for variances in air quality standards based on economic and technological feasibility. The Environmental Protection Agency (EPA), under Administrator Russell E. Train, had issued regulations that permitted states to adopt less stringent pollution control measures if they could demonstrate that stricter controls were economically or technologically unfeasible. However, this was challenged by the Natural Resources Defense Council (NRDC). The NRDC argued that such allowances violated the intent of Congress when it passed the Clean Air Act - to protect public health with an adequate margin of safety regardless of cost considerations. In a unanimous decision, the Supreme Court sided with EPA's interpretation allowing for some flexibility within state implementation plans regarding emission limitations based on technology availability and economic practicability while still achieving national ambient air quality standards set by EPA itself.

In the dissenting opinion for Train v. Natural Resources Defense Council, Inc., Justice William O. Douglas argued that the majority's decision to allow variances from pollution standards was not in line with the intent of Congress when it passed the Clean Air Act Amendments of 1970. He believed that these amendments were designed to be a strict mandate for cleaning up air pollution by specific deadlines, without exceptions or extensions. The majority's interpretation allowed for more flexibility and discretion on part of both state and federal agencies than he thought was intended by lawmakers. In his view, this could potentially undermine efforts to improve air quality as quickly and effectively as possible.