Decades-old trains, passenger crowding, forecasts of a $1.3 billion maintenance backlog and a surge in demand for services will heap pressure on Sydney's already stretched rail network as the city balloons to 6 million people over the next decade, an internal government report reveals.

The report, marked "Cabinet in confidence" and completed in September, underscores the need for the state government to inject even more funding into the backbone of the public transport system as an extra 1.4 million people are forecast to call the city home by 2031.

Sydney's rail network is under pressure from a surge in demand for services. Credit:Peter Rae

The Sydney Trains report, obtained by the Herald using freedom of information laws, reveals a backlog in deferred maintenance across the rail network is forecast to surge to $1.3 billion by the end of the decade, from $419 million last year, without a funding boost.

Despite recently retiring old trains dubbed "sweat sets", almost one-quarter of the state's passenger trains are still more than 30 years old, and nearly half of the fleet is more than two decades in age.