FUN FACTS ABOUT THE HERTS LOOP TRAINS!: An interesting fact is that the Moorgate to Drayton Park segment of the Hertford Loop line is underground and uses third rail DC (750V). Whenever trains stop at Drayton Park from either direction, you’ll always hear an electric sizzle followed by the engine cutting out, and the lights on board the train will black out temporarily, as it being switched to or from overhead AC (250kV). Due to the gradient of the train and timing in which the driver conducts the switch, the electrical switch is usually more apparent when coming down from the North to Moorgate.
Most of the trains I’ve taken have involved only one railway electrification system, a multi-system rail like this usually only occurs where trains cross a boundary, such as a national boundary, where two different countries have implemented different electrification systems.