The $2.5 billion rebuild of the Dandenong rail line must be expanded to include two extra tracks for express trains, an analysis has found, or rail passengers in Melbourne's south-east will be condemned to ever-slower train trips.

A group of transport experts has warned the Andrews government that Melbourne's busiest railway line will need four tracks in the near future to prevent travel times blowing out to unattractive levels as ballooning patronage creates slower travel times to and from the suburbs.

The Dandenong rail line will need extra tracks if commuting times are not to become unpalatably slow. Credit:Jason South

The Dandenong line is Melbourne's most congested. Its two tracks are shared by Metro's Pakenham and Cranbourne trains, V/Line's Gippsland services and freight trains. Levels of peak-hour overcrowding are consistently worse than on most other lines.

Rail commuters from Pakenham, Cranbourne and Gippsland also endure the slowest train travel times on the Metro network. The commute between Pakenham and the CBD averages 73 minutes, and it averages 61 minutes between Cranbourne and the city. The Gippsland line is V/Line's least reliable service because trains routinely get stuck behind Metro trains, taking about 46 minutes at an average speed of just 39 km/h.