Expansion of the railroad south through St. Clair and terminating at Marine City, 19.1 miles (30.7 km) from Port Huron,[4] was completed in 1918.

[6] The major industries located on the Port Huron & Detroit Railroad early in its history was the Marysville Power Plant, Pressed Metals, The St. Clair Rubber Company, Wills Sainte Clair Inc. (an automotive manufacturer), Diamond Crystal Salt, McLouth Shipyard and Independent Sugar Company.

In 1930, the unused spur track to the sugar beet plant in Marine City became the connection with the seven-mile Algonac Transit Company, an industrial railroad created at that time by the boat manufacturer Chris Craft in order to have a rail connection to its plant in Marine City.

[9][10] William N. Boyd, general superintendent and chief engineer of the PH&D, invented a wheel flange lubricator for these locomotives that was then adopted by other railroads.

[13] On December 13, 1984, the Port Huron & Detroit Railroad was sold by the Duffy family, who owned it since 1922, to the Chesapeake and Ohio Railway (at that time controlled by CSX Corp.).

[3] The locomotive used on the last train, number 60, was pulled in to the Port Huron roundhouse that evening for a closing ceremony and farewell party.