This is a cockney rhyming slang rhyme, it's referring to a section of the train line between Stevenage and Royston which is infamously known as 'The Street Car line'. It was named after Roderic Street and Procter Car who were two convicted criminals who would frequently travel the'streetcar line' as they had a habit of robbing passengers on board trains. They were then jailed in 'choke' or 'Chokey' as it is more commonly called.
