Sound Transit has placed new Sounder commuter rail cars built by Alstom into revenue service, marking the first passenger-carrying milestone of a fleet programme the two organisations marked with a joint announcement.

The entry into service means the cars have completed testing and acceptance and now operate in scheduled trains on the Sounder network, which runs diesel multiple-unit-style push-pull commuter services between Seattle and destinations including Tacoma and Everett in Washington state's Puget Sound region.

Alstom and Sound Transit described the occasion as a celebration of the delivery milestone, with the supplier confirming the vehicles are now carrying passengers rather than remaining in the commissioning and trial-running phase that precedes revenue operation.

For riders, the immediate effect of the new cars is additional and modernised capacity on a corridor where Sound Transit has been working to expand its regional express and commuter rail offer alongside its expanding light rail network.

The announcement comes from Alstom, and the operational claims — that the cars have entered revenue service — reflect the supplier's account of the programme status. Sound Transit's fleet plans call for continued replacement and expansion of the Sounder fleet as ridership recovers and the agency extends service under its ST3 programme of regional transit investment.

Further cars from the order are expected to follow the first sets into service as the fleet ramps up, with the full deployment of the new vehicles set to shape capacity on the Sounder South and Sounder North lines in the years ahead.