Rail passengers heading into the CBD in January are warned of significant changes to their travel, including extended journey times, with the City Loop closed for two weeks for upgrade works.

Flagstaff, Melbourne Central and Parliament stations are currently closed until the last service on Sunday, January 15.

There will be no City Loop services and trains will be operating directly to and from Flinders Street or Southern Cross station.

Buses will be replacing trains on the Hurstbridge Line between Parliament and Heidelberg on weekdays, and on the Alamein Line between Camberwell and Alamein on weekdays and weekends to allow the works to be completed safely.

Passengers who usually get off the train in the City Loop are encouraged to complete their journeys from Flinders Street or Southern Cross stations by walking or tram.

Larger trams with extra capacity will be available to help people get around the CBD.

Directional signs will be installed and customer service staff will be on hand to help people get where they need to go.

During this time, crews will be working around the clock to upgrade smoke detection, extraction and sprinkler systems at Flagstaff, Melbourne Central and Parliament stations, as well as decommissioning existing infrastructure.

Passengers should visit ptv.vic.gov.au/disruptions/city-loop-closure for more information and to plan their journey.

V/Line services on the Traralgon and Bairnsdale line will be replaced by road coaches from Wednesday, January 4, until the end of the month.

The situation comes as works on the Metro Tunnel Project, Gippsland Line Upgrade and V/Line maintenance continue.

V/Line is also completing works along the Traralgon Line between Tynong, Garfield, Bunyip, Drouin, Warragul, and Moe stations.

Due to the nature of these works, there will be additional light, noise, and dust present each day from January 4 to 29, as well as each night between 7pm and 7am from January 18 to 23.