Which London Underground line was formerly called the Metropolitan District Railway?

The District line was formerly called the Metropolitan District Railway.

The Metropolitan District Railway was established in the nineteenth century to build a railway around the southern and western sides of central London. Its first section opened in 1868 between South Kensington and Westminster. It later extended westward, eastward, and into suburban areas.

The railway originally worked closely with the Metropolitan Railway, helping create the route that became the Inner Circle. Despite their shared history, the two companies were separate and competed for passengers and routes. The District Railway was electrified in the early twentieth century and became part of the Underground system operated by London Transport.

The modern District line is therefore a descendant of the Metropolitan District Railway, not the Metropolitan Railway. That similar wording is the main source of confusion between the two names.