The MBTA has contracted with Springfield-based CRRC to design and manufacture a fleet replacement encompassing 152 new Orange Line vehicles and 252 new Red Line vehicles. MBTA photo

The first brand-new Orange Line cars will enter passenger service Wednesday, MBTA officials announced, marking a key step in an ongoing project to replace the entire fleet with modern vehicles.

MBTA General Manager Steve Poftak told the T’s oversight board Monday that the first six cars – whose introduction has been delayed several times – would roll out on the tracks midday on Wednesday.

“They are brand new cars, they are brand new technology, they have a lot to offer to our customers,” Poftak said.

The first new Orange Line vehicles were initially scheduled to begin running in January, but the date was pushed back to the spring and then to the summer amid challenges with updating a key software system.

Another set of new cars will launch in September, Poftak said. The full 152-car, Springfield-built fleet is scheduled to be delivered by December 2021.

The MBTA is also planning a fleet of 252 new cars on the Red Line by 2023, and officials say the two projects together will add about 85,000 seats to the transit network.