What was the first London Underground line to use trains powered by electricity?

The City & South London Railway was the first London Underground line to use electrically powered trains.

It opened in 1890 between King William Street in the City and Stockwell in south London. The railway was built as a deep-level tunnel route and used electric locomotives to haul passenger carriages. Its opening marked a major technological change from the steam-hauled Metropolitan and District railways.

The original electric locomotives were later replaced by electric multiple-unit trains as the railway was extended and modernised. The line eventually became part of today’s Northern line, although much of its original route and station infrastructure was altered.

This fact is distinct from the first deep-level electric railway: the City & South London Railway is that railway, but the wording here focuses on its use of electric train power. It is also different from the Central London Railway, which opened later in 1900.