The longer platforms will cater for the new high-capacity metro trains that will first travel on the Cranbourne-Pakenham line. Work will begin to extend the platforms later this year.

In total, 18 stations on the line will have the longer platforms, including five stations being rebuilt as part of the level crossing removal program.

The new trains, which will be 20 per cent longer than the existing fleet, are currently being built.

The upgrades to the line include the rebuilding of overhead power lines and constructing or upgrading substations. A new depot in Pakenham East will also be built with maintenance facilities and stabling for up to 30 trains.

The Cranbourne-Pakenham line will eventually form part of the Melbourne Metro Tunnel project that will run through five new underground stations. It will join the Cranbourne-Pakenham and Sunbury lines.

Public Transport Minister Jacinta Allan said the new trains would initially run on the Cranbourne-Pakenham line but would travel to Sunbury when the Melbourne Metro project was completed.

However, similar upgrades will need to be carried out on the Sunbury line to accommodate the new trains.

"We will need similar platform extensions and power and signalling upgrades on the Sunbury part of the network," Ms Allan said.

Stations to receive extended platforms include South Yarra, Caulfield, Malvern, Oakleigh, Westall, Springvale, Dandenong, Merinda Park, Cranbourne, Hallam, Berwick, Officer and Pakenham.

Stations will be rebuilt at Carnegie, Hughesdale, Murrumbeena, Clayton and Noble Park as level crossings are removed.

Other stations on the Cranbourne-Pakenham line and the City Loop are already long enough for the new trains.

The new trains are expected to begin running in 2019. The government has promised that all 65 high-capacity trains will be ready for the opening of the Melbourne Metro tunnel in 2026.

Premier Daniel Andrews said it was impossible to "retire" old trains without building new models.