The Canadian Pacific 6591 S3 diesel-electric locomotive was built by Montreal Locomotive Works in 1957. It was used as a switcher in the Smiths Falls area, moving short trains to nearby terminals as well as assembling cars to make up trains in the CPR Smiths Falls yards.
CP 6591, a switcher diesel, has only 600 hp compared to the 4,400 hp that road locomotives have today. It was assigned to Smiths Falls and used in the freight yard. CP 6591 is believed to be the last engine to go through the Brockville Tunnel. It was retired in 1982. It was donated to RMEO in 1985 and was repainted.