A RADICAL plan to convert large parts of the CityRail network to single-deck high frequency trains is closer to winning state government acceptance.

Detailed reports show senior bureaucrats have been told a multibillion-dollar overhaul of the system would be the cheapest and most efficient way to add capacity to CityRail during the next 25 years.

The Herald has obtained documents that include cost estimates of the options.

One estimate shows costs of between $26 billion and $37 billion for adding capacity during the next 25 years. Without extra capacity through the CBD, trains will soon be so crowded in peak hours that services will need to be cut.