Major corridors in Sydney's rail system will be overloaded within five years, the government's own analysis shows, with commuters throughout the city regularly confronting trains too crowded to board.

After decades of urban development and underinvestment, the Western Line between the CBD and Penrith is looming as the big crisis point for the city's train system, regardless of which party wins government next month.

Confidential analysis shows the government's own forecasts predict commuters from as far as Blacktown will be unable to get a seat into the city and trains will be overloaded by the time they get to Parramatta.

Documents obtained using freedom of information laws also show the government is assuming that by 2021 all seats will be filled on morning Illawarra Line trains from as far south as Sutherland, and there is a risk commuters from stations such as Oatley and Mortdale will be unable to board trains.

Inner west trains could also be too full to pick up passengers at Newtown and Macdonaldtown, the analysis shows.

And the document shows the number of people forced to change trains to get to their destination is expected to increase about 80 per cent by 2021, putting huge pressure on already-overcrowded city stations.

The state government's election pitch is to fund a massive expansion in the rail network using the proceeds of the privatisation of electricity assets. The Labor opposition has not stated its transport policy.

The government's expansion would extend the $8.3 billion North West Rail Link,  which is under construction, through the city via a new harbour crossing and connect it to the Bankstown Line at Sydenham.

But this series of projects, to cost about $20 billion, would not address the most congested artery in the rail network.

Transport for NSW's Sydney's Rail Future 2021 Demand Assessment shows the Bankstown Line has the most spare capacity of all big rail lines between the city and Sydney's west.

The government says that by removing Bankstown Line trains from the City Circle, and extending them across the harbour on the new line, space would emerge for extra services on the Inner West line and from Campbelltown and East Hills.

But this would do little to enhance capacity on the Western Line, which emerges clearly as the biggest crisis point in Sydney's train system.

The document obtained by Fairfax Media shows patronage on the Western Line is expected to increase 4.5 per cent a year until 2021, the fastest growth on the network. This is followed by the Northern Line (3.7 per cent) between Epping and Strathfield and the Airport & East Hills Line (3.3 per cent).

The documents show  the average load on Western Line services arriving at Parramatta would be more than 1400 passengers a train. It is difficult to run services on time when trains carry more than 1200 passengers because people take a long time to get on and off carriages in the crush.

Punctuality figures released last week show the Western Line, Sydney's busiest, is already struggling to cope. The line recorded the city's lowest punctuality figure last year at 92.2 per cent, just above the government's 92 per cent target and below the overall rate of 94.7 per cent.

As well as the new harbour crossing, slated to cost more than $10 billion, the government has promised more than $1 billion of works for a "western Sydney rail upgrade", which it says should allow more trains to run on the Western Line.

Transport for NSW says the upgrades will include new signalling systems between Westmead and Granville, track improvements at Homebush, new "turn-backs" at St Marys, and a new train control system. But it is understood there is still much scepticism within the department about whether the upgrades will produce the stated benefits.

Transport Minister Gladys Berejiklian said the western Sydney rail upgrades, combined with the second harbour crossing, would produce 60 per cent more capacity on the overall rail network.

"One thing is very clear – the do-nothing scenario is simply not an option," Ms Berejiklian said. "Labor left this city in a complete mess and that is why the NSW government laid out its plans in Sydney's Rail Future and continues to deliver on its promises."

Labor leader Luke Foley does not yet have a transport policy, though he has said he would announce one this month.

The analysis obtained by Fairfax Media includes the operation of the North West Rail Link, scheduled to open in 2019. Transport for NSW says passengers who use the Western Line will shift to the new north west stations.

"Thousands of customers in the morning peak will stop using the Western Line and instead use the North West Rail Link from places like Seven Hills, Blacktown and the Richmond branch – because it will be faster to get to where they're going, particularly when the second harbour crossing is complete," a spokeswoman said.

And the analysis predicts a huge increase in the number of people changing from one train to another. By 2021, it is expected that there will be 107,000 "interchange" movements, up from about 60,000 in 2011.