Crossrail is a major new railway link being built under central London. The name refers to the first of two routes which are the responsibility of Crossrail Ltd, the other being the proposed Chelsea–Hackney line. It is based on entirely new main-line gauge east-west tunnels from Paddington in the west to beyond Whitechapel in the east. The company Cross London Rail Links (CLRL) (now known as Crossrail Ltd.) was formed in 2001 to deliver the scheme. The project was approved in October 2007, and the Crossrail Act received Royal Assent in July 2008.

Ten-car trains will run at frequencies of up to 24 trains per hour (tph) in each direction through the central tunnel section.

The original plan was that the first trains would run in 2017. In 2010 a Spending Review saving over £1 billion of the £15.9 billion projected costs meant that the first trains are now planned to run on the central section in 2018. It has become Europe's biggest construction project.

The concept of large-diameter railway tunnels crossing central London to connect main-line train services across the capital has its post-war origin in the 1943 County of London Plan and 1944 Greater London Plan by Sir Patrick Abercrombie. These led to a specialist investigation by the Railway (London Plan) Committee, appointed in 1944 and reporting in 1946 and 1948.Route A would have run from Loughborough Junction to Euston, replacing Blackfriars bridge and largely serving the same purpose as today's Thameslink Programme. Route F would have connected Lewisham with Kilburn via Fenchurch Street, Bank, Ludgate Circus, Trafalgar Square, Marble Arch and Marylebone. This was seen as a lower priority than Route A, although another proposal, Route C, was the only one eventually built, as the Victoria line.