Line M4 (Rijndam)
Rijndam Airport - Transferium Rijndijk
The Line M4 connects the city with Rijndam Airport. It runs above ground from Transferium Rijndijk to Koning Willem I, then underground to Noorderpoort, and then continues above ground to Rijndam Airport. Shortly before reaching the airport, the line returns underground, to terminate under the Airport terminal building containing terminals 1,2 and 3 (passengers for Terminal 4 can change here for a shuttle service). As with line M1, the section outside of the city after Noorderpoort uses a converted railway line. It used to have overhead wires and ZUB train protection, but (in 2020) the line was converted to 3rd rail and ATB in order to facilitate to use of rolling stock without pantograph on line M4.
Line M4 has its own depot at the airport, using the site of a former underground cargo terminal.