This is a case of conflicting sources. Certain sources, including one poster in the station on the set, describe the "eastenders line" from Walford East to "Up west". In which case, they can use what they want.
They must have modernised anyway, I'm sure the stock used to have motors that sounded like conventional 3rd rail trains (I have no idea what stock they used to use the sounds of, so that is my best try at a description!).