 During the 1870s, the population of McGregor exploded to over 5,500 as the city became the busiest shipping port west of Chicago.In 1874, the system of ferrying railroad cars across the river between North McGregor and Prairie du Chien, Wisconsin, was brought to an end when Prairie du Chien businessman John Lawler commissioned the construction of a permanent pontoon bridge to connect the two cities' rail lines.As the need for men to disassemble and ship trains across the river disappeared, the city's population began to decline.