Transportation Line v. Cooper was a case heard by the United States Supreme Court in 1878. The case involved a dispute between the Transportation Line and the Cooper family over the ownership of a steamboat. The Transportation Line had purchased the steamboat from the Coopers, but the Coopers later claimed that the sale was invalid because they had not received full payment. The Supreme Court ruled in favor of the Transportation Line, finding that the Coopers had received full payment for the steamboat. The Court held that the Coopers had accepted the payment and had no right to reclaim the steamboat. The Court also held that the Coopers had no right to sue for damages, as they had already received full payment for the steamboat. The Court's decision in Transportation Line v. Cooper established that a sale is valid and binding once full payment has been made, even if the seller later claims that the sale was invalid. This decision has been cited in numerous subsequent cases involving disputes over the validity of sales.

In Transportation Line v. Cooper, the Supreme Court was asked to decide whether a state law that required railroad companies to pay for damages caused by their negligence violated the Constitution's Commerce Clause. The majority opinion held that it did not violate the clause because it applied equally to all railroads regardless of where they were located and thus did not discriminate against interstate commerce. However, Justice Field dissented from this decision on two grounds: first, he argued that since the law only applied within one state, it could be used as an instrument of discrimination against out-of-state railroads; second, he argued that even if there was no intent to discriminate in this case, such laws could easily be abused in future cases and should therefore be struck down as unconstitutional.