Vote trading among lawmakers (logrolling) can enable political rent-seeking but is difficult to identify. To achieve identification, we explore the rules governing voting for railway projects in the U.K. Parliament during the Railway Mania of the 1840s. Parliamentary rules barred MPs from voting directly for their interests. Even so, they could trade votes to ensure their interests prevailed. We find that logrolling was significant, accounting for nearly one-quarter of the railway bills approved. We also quantify a negative externality to society from logrolling ranging from 1/3 to 1 percent of contemporary GDP.