Crossrail is a railway construction project underway mainly in central London. Its aim is to provide a high-frequency suburban passenger service crossing the capital from west to east, by connecting two major railway lines terminating in London, the Great Western Main Line and the Great Eastern Main Line. The project was approved in 2007, and construction began in 2009 on the central section and connections to existing lines that will become part of the route, to be branded the Elizabeth line.

The main feature of the project is the construction of a new railway line that will run underground from near Paddington Station via central London and Liverpool Street Station to Stratford. Another almost entirely new line will branch off the main line at Whitechapel in east London. It will run to Canary Wharf, cross the Thames and connect with the North Kent Line at Abbey Wood in southeast London.

New nine-carriage Class 345 trains will run at frequencies in the central section of up to 24 trains per hour in each direction. At each end of its central core services it will divide into two branches: in the west to Reading and to Heathrow Central; in the east to Abbey Wood and to Shenfield. In May 2015, services on a section of one of the eastern branches, between Liverpool Street and Shenfield, were transferred to TfL Rail; this precursor service also took control of Heathrow Connect in May 2018 and the Paddington to Reading line in December 2019.

The Elizabeth line will be operated by MTR Corporation (Crossrail) Ltd as a London Rail concession of Transport for London, in a similar manner to London Overground. TfL's annual revenues from the line were forecast in 2018 to be nearly £500 million per year in 2022/23 and over £1 billion per year from 2024/25.

The total estimated cost has risen from an initial budget of £14.8 billion to £18.7 billion, as of August 2020. Originally planned to open in 2018, the project continues to be delayed. The central section is planned to open in the first half of 2022. The COVID-19 pandemic had caused Crossrail Ltd to pause physical works temporarily on all sites; these have subsequently resumed but with fewer than half the number of construction workers previously allowed on site.