The British Rail Class 71 was an electric locomotive used on the Southern Region of British Railways . Unlike most other Southern Region electric locomotives ( such as the class 73s & 74s ) they could not operate away from the electrified ( 750 V DC third rail ) system . The British Rail Class 71 was an electric locomotive used on the Southern Region of British Railways , unlike most other Southern Region electric locomotives ( such as classes 73 and 74 ) they could not operate away from the electrified ( 750 V DC third rail ) system .