Record growth in passengers on Sydney’s stretched rail network has spurred the Berejiklian government to order an extra 17 new trains to cope with the "off-the-charts" demand.

The new batch of Waratah trains, which will be built in China, will cost the state about $900 million, which includes their ongoing maintenance. Once off the production line, the first of the 17 Waratah B-sets is due to begin running on the city's suburban network in August 2020.

The latest order comes as Sydney Trains has pressed into service half of the 24 Waratah B-sets ordered several years ago. The latter will allow the rail operator to retire four-decades-old S-Set trains – dubbed “sweat sets” because of their lack of airconditioning – by mid-year.

The NSW government has ordered another 17 Waratah trains, the first of which is expected to begin services in August next year. Credit:Brendan Esposito

Transport Minister Andrew Constance said the investment in the trains – noticeable for their orange driver cabins – was needed because of the rapid growth across the rail network.