Ex parte Baltimore & Ohio Railroad Company was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue an injunction against a railroad company. The case arose when the Baltimore & Ohio Railroad Company (B&O) sought to construct a bridge over the Ohio River. The state of Ohio objected to the construction of the bridge, arguing that it would interfere with the navigation of the river. The state court issued an injunction against the railroad company, preventing it from constructing the bridge. The Supreme Court held that the state court did not have the authority to issue the injunction. The Court reasoned that the power to regulate interstate commerce was vested in Congress, and that the state court was not authorized to interfere with the exercise of that power. The Court further held that the state court's injunction was an unconstitutional interference with interstate commerce. The Court's decision in this case established the principle that state courts cannot interfere with the exercise of Congress' power to regulate interstate commerce. This principle has been applied in numerous cases since then, and remains an important part of the law today.

In Ex Parte Baltimore & Ohio Railroad Company, the Supreme Court was asked to decide whether a lower court had correctly dismissed an injunction against the railroad. The majority opinion held that the lower court did not err in dismissing the injunction because it found no evidence of fraud or bad faith on behalf of the railroad. Justice Field dissented from this decision and argued that there were sufficient facts presented by plaintiff to support their claim for injunctive relief. He believed that if these facts were considered, then a different result would have been reached by both courts. Furthermore, he argued that even though there may have been some technical errors made during proceedings before trial, they should not be used as grounds for dismissal when considering such serious matters as injunctions which could affect property rights and other interests of parties involved in litigation.