Passenger crowding on trains travelling along Sydney's western rail spine to the city during the morning peak has worsened significantly over the past year, underscoring the need for major investment in the ageing network.

Half of suburban lines have at least some trains unable to fit more passengers on during the busiest hour of the peak from 8am to 9am, figures from the state's transport agency show.

Trains on the busiest line, the T1 Western, recorded average passenger loads of 150 per cent during the morning peak in March, up from 139 per cent a year earlier.

Major relief to surging demand on the Western Line is a decade away when a $20 billion metro rail line carrying single-deck trains between central Sydney and Parramatta is due to open.