The Alleghany was a landmark Supreme Court case that established the principle of federal preemption. The case involved a dispute between the state of Pennsylvania and the federal government over the right to tax a railroad. The state of Pennsylvania had imposed a tax on the railroad, but the railroad argued that the tax was unconstitutional because it interfered with the federal government's power to regulate interstate commerce. The Supreme Court agreed with the railroad, ruling that the state tax was unconstitutional because it interfered with the federal government's power to regulate interstate commerce. The Court held that the federal government had the power to preempt state laws that interfered with its power to regulate interstate commerce. This ruling established the principle of federal preemption, which has been used to limit the power of states to regulate certain areas of commerce. The Alleghany case is still cited today as an important precedent in the area of federal preemption.

In The Alleghany, the Supreme Court was tasked with determining whether a railroad company had the right to build its tracks across state lines. Justice Field wrote a dissenting opinion in which he argued that Congress did not have the power to grant such authority and that it should be left up to each individual state. He asserted that allowing railroads to cross state boundaries would create an imbalance of power between states, as some states may benefit more than others from increased transportation opportunities. Furthermore, he argued that this could lead to monopolies or other unfair practices by large companies who are able to take advantage of their ability to move goods quickly across multiple jurisdictions. Ultimately, Field concluded that if Congress wanted railroads crossing different states’ borders they should pass legislation specifically granting them permission rather than relying on general laws meant for all types of interstate commerce.