[1] Operated by Metro Trains Melbourne, it is the city's fifth longest metropolitan railway line at 40.3 kilometres (25.0 mi).

[6] The line was progressively electrified over time, with electrification to St Albans in 1921, to Sydenham in 2002, and finally to Sunbury in 2012.

[7][8][9] Since the 2000s, due to the heavily utilised infrastructure of the Sunbury line, improvements and upgrades have been made.

[12] On 27 January 2002, electrification of the St Albans line was extended along the regional V/Line tracks to Sydenham.

Proposals for an extension of electrified metropolitan services to Sunbury have dated as far back as the 1969 Melbourne Transportation Plan.

Concerns included a less comfortable journey, the removal of toilet facilities, the presence of a conductor, potential overcrowding, and congestion at level crossings due to the increase in the number of services.

[15] Bendigo line users also raised concern that electrification to Sunbury would slow down their trains to Melbourne.

[19] Works delivered as part of the project include the installation of five new electrical substations: one in Delahey (near Watergardens station),[20] one in Calder Park, rail yard[21] two in St Albans,[22][23] and one in Albion;[24][25] ten more across the line were upgraded along with improvements to overhead wiring and signalling equipment to enable the use of "high capacity signalling" (a form of communications-based train control[26]) in the Metro Tunnel.

Raised boarding pads were installed at Sunbury, Diggers Rest, Watergardens, Keilor Plains, Albion, Sunshine, and Footscray stations to improve wheelchair accessibility.

[31] In 2017, the Metro Tunnel project began construction, consisting of twin 9-kilometre (5.6 mi) tunnels bored underground between South Kensington and South Yarra stations, with five new stations along the route: Arden, Parkville, State Library (connecting to Melbourne Central), Town Hall (connecting to Flinders Street), and Anzac.

[32][33] Another crossing was removed by elevating a section of the Melton Highway in Sydenham onto a bridge above the rail line in 2018.

[38] Train services on the Sunbury line are also subjected to maintenance and renewal works, usually on selected Fridays and Saturdays.

The route is 40.3 kilometres (25.0 mi) long and is fully doubled tracked from Flinders Street to its termnius.

[46] Exiting the city, the Sunbury line traverses mainly flat country, except on the west bank of the Maribyrnong River where it requires some moderately heavy earthworks.

The Sunbury line uses Comeng and Siemens Nexas electric multiple unit (EMU) operating in a split six-car configuration and, for the two morning peak services, the High Capacity Metro Trains (EMU) operating in a seven-car configuration, with three doors per side on each carriage and can accommodate of up to 1,380 passengers in each train-set.

The primary rolling stock featured on the line is the Comeng (EMU), with three doors per side on each carriage.

[58] As of October 2023, two HCMT services run per day on the Sunbury Line, with more to be gradually introduced in the coming years.

This includes St Albans and Ginifer stations, which were rebuilt as part of the Level Crossing Removal Project; and Sunbury, Diggers Rest, Watergardens, Keilor Plains, Albion, Sunshine, and Footscray stations which had raised boarding pads installed to improve wheelchair accessibility as part of the Sunbury Line Upgrade.