A Transport for NSW (TfNSW) document, obtained using freedom of information laws, forecasts what the NSW rail network will look like in 2021.

The report shows the government is expecting morning peak hour trains on some lines will be completely full well before they reach their destination.

The report, Sydney Rail Future 2021 Demand Assessment on the 2019 Reference Case, defines the total capacity of a rail carriage as 150 people - 112 seated and 38 standing.

The TfNSW analysis forecasts overcrowding will not be a problem for commuters on the South Coast line in 2021.

Thirroul will be the busiest station, with more than 400 people catching the train.

Overcrowding is predicted to surface once trains on the South Coast line switch to the Illawarra line.

The report predicts that by the time the trains reach Sutherland there will be no seats left, which means those travelling from there to Central will be standing for about half an hour.

But at least they could get on - there is the chance commuters at stations between Oatley and Hurstville may not even be able to board the train at all due to overcrowding.

The passenger load is not predicted to diminish until the train leaves Central, while those commuters getting off there can expect a far busier station.

The 2021 predictions show passenger movements in the morning peak will rise by almost 40 per cent on 2011 figures - from 38,000 to 53,000.

The report also mentions passenger loads have jumped 39 per cent since March 2010.

Measured between Helensburgh and Waterfall, the morning peak passenger load from the four city-bound trains in March 2010 was 1460.

But four years later it had climbed to 2027.

On Monday, Transport Minister Gladys Berejiklian said the report made it clear that action had to be taken.

"One thing is very clear - the do nothing scenario is simply not an option," Ms Berejiklian said.

‘‘The NSW Government is committed to ensure capacity on the South Coast Line grows with customer demand – we have already boosted services in the recent timetable and we are delivering a brand new, modern intercity fleet.

‘‘These are the sort of improvements Illawarra customers missed out on under Labor as only the Liberal Government has shown we are prepared to invest to grow public transport.’’

A spokeswoman for TfNSW said the evidence in this report was why a plan was in place to ‘‘transform and modernise the rail network so that it can grow with our population and provide more services, including from the South Coast’’.

‘‘Work is already underway to improve reliability and increase capacity, for example, a brand-new intercity fleet is on the way for South Coast customers,’’ the spokeswoman said.

‘‘While it is not unreasonable to expect that large numbers of people will travel to and from the Sydney CBD in the peak, our aim is to make sure customers with the longest journeys from places such as the South Coast will get a seat for the duration of their trip in the morning peak.

‘‘We will continue to monitor demand and plan for growth to ensure public transport services are delivered when and where they are needed.’’