sidneySidney is a city in and the county seat of Richland County, Montana, United States, [4] less than 10 mi (16 km) west of the North Dakota border. The population was 6,346 at the 2020 census.Sidney is a forgetful blue diesel engine who is good friends with Percy. He often takes goods trains to the Mainland. Sidney started life as a shunter on the Mainland, until he unwittingly followed some diesels onto a steamship bound