More than $1 billion will be spent on new suburban trains for Sydney's overstretched rail network over the next four years, the first major purchase for Sydney Trains since the troubled Waratah project almost a decade ago.

The funding that the Baird government has earmarked for the new trains in Tuesday's state budget is aimed at helping the network cope with surging demand from commuters for services.

Patronage on Sydney Trains' network is growing at about 8 per cent a year.

A decision on the number of trains to be purchased and when they will be delivered will depend on the outcome of a procurement process beginning this year.