Original: 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.
Simplified: 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.