Ordered by CDPQ Infra in 2018, the Alstom Metropolis was retained to serve as the rolling stock for the REM. In total, 212 cars were ordered, operating in four-car during peak and two-car units during off-peak hours. The trains were assembled in India and started to be delivered in October 2020. While used for the REM, the Metropolis can also be found around the world and is the basis for the rolling stock in many transit networks including Paris, Singapore, Sydney, Dubai, and Buenos Aires. The platform is highly customizable and designed to meet the operational needs of operators throughout the world.

The Alstom Metropolis is a light rail, high-floor trainset that is 100% electrified and benefits from regenerative breaking, full-LED lighting and modern HVAC equipment. Each pair of cars is semi-permanently coupled, with four-car units consisting of two pairs attached with an exterior coupling mechanism. The consists are powered by an overhead catenary that provides 1,500V DC, with one motor on each axle. Peak speeds of 100 km/h are expected on some segments, with an average speed of 51 km/h. While the trains are larger than the MPM-10 Azur rolling stock, they use standard gauge trackage. Once closed, the wide doors will be flush with the side of the train to reduce drag and improve aesthetics.

The trains are fully automated and rely on an Alstom-designed Automatic Train Control (Urbalis 400 CBTC): as such, there will not be a cab in the vehicles, leaving the ends of each car fully accessible to passengers. Should Automatic Control be disabled, or manual movement of the train be required, a "hidden" control panel can be found under the covers at the end of each car and is locked out to avoid accidental intervention.

Due to their high-floor nature, the trains have an emphasized focus on accessibility with level-boarding at all stations, level-floor throughout and wide-open areas to facilitate movement in the train. Reserved spaces are found throughout to accommodate passengers with mobility needs. Before production started, a life-size model of the rolling stock was presented to customers in Montreal and feedback was used to improve the passenger experience. The choice of colours and contrast between the elements was done to improve accessibility to users with visual difficulties.

To enhance the passenger experience, the Metropolis was considerably customized for the REM. The visual identity of the trains, including the cab-end and accent colours, emphasizes the various elements of Montreal. The three designs, Saint-Laurent, Mont-Royal and L'Expo were presented to the public who then offered their feedback in 2018. 42% voted in favour of the St-Laurent Design, which was used to finalize the identity of the rolling stock. On the interior, maps of the various branches, a screen indicating the next stop and destination, Wi-Fi, heated floors and capacity indicators (using sensors integrated into the hydraulic suspension of the cars) allow for a seamless passenger experience. Of the projected 600 passengers in a 4-car train, 128 will be seated. The seats feature an increased side-bolster to counteract the sideways alignment of passengers compared to the train movement.

Located south of the intersection of Highway 10 and 30, Brossard Maintenance and Storage Facility (MSF) is the primary facility for the ongoing maintenance and storage of the Alstom Metropolis rolling stock. The facility also contains the main control centre for all operations and is staffed 24/7.

When trains end service at Brossard station, they will continue onward to enter the facility and, based on the operational needs, either be sent to the storage or maintenance hall.

To ensure the cleanliness of the rolling stock, an automated "car-wash" is found at the entrance of the facility where trains can be automatically cleaned before going into storage.

Whenever trains are not in-service, the storage yard is a covered facility part of the MSF. It contains 9 tracks that can accommodate 54 double cars and 6 trains per track. including all the ones needed to operate the South Shore branch once it opens. Each track is directly accessible from outside through an automated door.

As the trains are fully-automated, their movement within the storage yard is also automated using a similar positioning system as found throughout the line. Inside the main hall, the trains are accessible from either side to allow for visual inspection of the rolling stock and can move on their own power. Various interlocking systems are in place to ensure the safety of staff navigating within the facility.

Composed of four tracks parallel to the Storage Hall, the Maintenance Hall is where the necessary work to maintain the fleet of Alstom Metropolis used by the REM. Given the nature of the work and the possibility of damage or issues, trains might operate in manual control to enter and exit this facility, a rare occurrence on the REM network.

The facility is equipped to perform preventive and corrective maintenance on-site for the duration of the contract between Groupe PMM (SNC Lavalin / Alstom Partnership) and CDPQ Infra. Work bays are found on two tracks, cranes allow for the separation of bogeys and cars, and jacks and elevated platforms allow access to all elements of the rolling stock.

As the REM will operate 20 hours out of every day, maintenance will be done on a rotating basis between the vehicles, ensuring that enough trains are available for the expected level of service and dynamic spares should any issue arise.