The order takes the total number of B-series trains ordered for Transperth to 66. With the delivery of the final train in 2016, the city's suburban train fleet will be expanded by 28% from 234 to 300 vehicles.
The additional trains will help to support the expansion of the network and rising passenger numbers, which have increased by 15% over the last three years.
State transport minister Mr Troy Buswell said this could be the last order for B-Series vehicles as he has asked the Public Transport Authority (PTA) to evaluate international rolling stock design with the aim of developing a specification for a new-generation suburban train.
Meanwhile, the PTA has signed a $A 19.7m ($US 20.6m) contract with Ansaldo STS to design, supply, and install signalling on the 7.5km extension of the Joondalup Line from Clarkson to Butler. The new line will open in 2014.