The network will be completed by 2020 and will cover 70 kilometres with 37 stations.
Qatar on Wednesday opened its first metro line, three years before the Gulf nation is due to host the 2022 FIFA World Cup.
The trains, which boast aeroplane-style seats, will run across the capital Doha, mainly underground.
Footage showed some of the first passengers riding the red line south. The full metro network will consist of three lines spanning 37 stations and is expected to reach completion by 2020, covering more than 70 kilometres (43.5 miles).
A green line between Al Riffa and Al Mansoura and a yellow line between Al Azizyah and Ras Bu Abboud will also be added.
Currently, the red line south will be open Sunday to Thursday from 8 am to 11 pm local time, not running on a Friday or Saturday, the local weekend.