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, and for this reason the line, except in very limited circumstances, does not operate on Sundays or public holidays. It is one of only two lines on the Underground network to run completely underground, the other being the Victoria line.

Printed in turquoise on the Tube map, it is by far the shortest line on the Underground network, being 2.37 km (1.47 miles) long, with an end-to-end journey lasting just four minutes. 

The Depot is entirely below ground and is situated beyond the Waterloo & City line platform end at Waterloo Underground station. Trains from Bank reverse at the depot before returning. It is one of the smallest depots on the Underground network and one of few to be completely underground. For larger items like rolling stock, lengths of rail and other heavy machinery, access to the depot can only be achieved via a shaft on Spur Road, requiring a large crane to do so.