The West Metro section has one underground metro depot, which has a surface area of 32,500 m2. The depot is located in Sammalvuori, Espoo.

There are two underground halls for depot operations – one is for the night-time storage of 20 train units, the other is for maintenance facilities. Two sections of the maintenance hall have been set aside for graffiti removal and lathes. The maintenance hall is approximately 300 metres long, and it is wide enough to accommodate three trains. With working spaces included, its width is roughly 25 metres. At the north end of the maintenance hall are technical spaces and an area for maintenance traffic.

The train storage hall is about 400 metres long and 26 metres wide. The depot will be heated entirely with geothermal heat and cooled using low-temperature groundwater.

The Sammalvuori consortium (Kalliorakennus-Yhtiöt Oy, EM Pekkinen Oy, Terrawise Oy and Aro-Systems Oy) implemented the excavation and construction contract for the Sammalvuori metro depot. The contract was started in 2016 and completed in 2020. Metropolitan Area Transport took the metro depot into use when commuter traffic on the Matinkylä–Kivenlahti section was opened to the public in December 2022. Metropolitan Area Transport owns the metro trains and is responsible for maintaining them.