The Waterloo & City line, colloquially known as The Drain, is a London Underground shuttle line that runs between Waterloo and Bank with no intermediate stops. Its primary traffic consists of commuters from south-west London, Surrey and Hampshire arriving at Waterloo main line station and travelling forward to the City of London financial district. For this reason, the line has historically not operated on Sundays or public holidays, except in very limited circumstances. Following the COVID-19 pandemic, the line is currently only open on weekdays. It is one of only two lines on the Underground network to run completely underground, the other being the Victoria line.

The London Underground is a rapid transit system serving Greater London and some parts of the adjacent home counties of Buckinghamshire, Essex and Hertfordshire in England.