The option, which comes from a contract for 40 vehicles awarded to Transtech in December 2011, is worth €100.5m, including €5.3m for spare parts and specialist maintenance equipment.
At 35m in length, the bidirectional vehicles will be significantly longer that the 27.6m-long Artics currently being built at Transtech ‘s Otanmäki plant for the metre-gauge network in Helsinki.
HKL says it expects to conclude negotiations with Transtech in the coming weeks and a prototype vehicle is due to be delivered in late 2018 or early 2019.
The 25km Raide-Jokeri line will link Keilaniemi, Otaniemi and Leppävaara in Espoo with Haaga, Pakila, and Oulunkylä in northern Helsinki before terminating at Itäkeskus in the east.
The 33-station line will closely follow the route of Line 550, Helsinki's busiest bus line with peak services operating at three-minute intervals.
The line is expected to open before the expiry of the current operating contract for the bus route in 2022.