Between Farringdon and Aldgate East it skirts the City of London, the capital's financial heart, hence the line's name.

[3] Starting in 2015, the signalling system was upgraded as part of a programme to increase peak-hour capacity on the line.

The first line built by the Metropolitan Railway (Met) was from Paddington to near Smithfield, near London's financial heart in the City; with gas-lit wooden carriages hauled by steam locomotives.

[6][7] Built on viaduct largely across open fields,[8] the line opened on 13 June 1864 with a GWR service from Farringdon[9] to Hammersmith,[10] services to Addison Road (now Kensington (Olympia)) on the West London Railway via a link at Latimer Road starting a few weeks later.

It had been intended to operate the new trains with four or six cars,[19] but after initial problems with the traction current only six-car formations were used.

[20] Services to Kensington (Olympia) via the curve at Latimer Road were suspended for the duration of World War II after bomb damage to the West London line in 1940.

[10][25] In 2003, the infrastructure of the Hammersmith & City line was partly privatised in a public–private partnership, managed by the Metronet consortium.

[26] The reconstruction of Whitechapel station for the new Crossrail line required the reversing platforms to be taken out of use and since December 2009 all services have been extended to Plaistow or Barking.

[32] The line enters a cutting just west of Paddington, with a cut-and-cover tunnel at the far eastern end of the platforms.

It meets the Circle and District lines from Bayswater at Praed Street Junction before passing through Edgware Road station in a cutting.

[38] Services are provided by S7 Stock trains, part of Bombardier's Movia family, with air-conditioning as the sub-surface tunnels (unlike those on the deep-level tube lines) are able to disperse the exhausted hot air.

[43] Traction voltage was increased in 2017, from the present nominal 630 V to 750 V to give better performance and allow the trains to return more energy to the network through their regenerative brakes.

It was planned that a new signalling system would be used first on the sub-surface lines from the end of 2016,[44] but signalling contractor Bombardier was released from its contract by agreement in December 2013 amid heavy criticism of the procurement process[45] and London Underground subsequently awarded the contract for the project to Thales in August 2015.

[46] With the introduction of S7 Stock, the track, electrical supply, and signalling systems are being upgraded in a programme planned to increase peak-hour capacity on the line by 27 per cent by the end of 2023.

[44][49] The rollout of CBTC has been split into sections, each known as a Signal Migration Area (SMA), and are located on the line as follows:[50]