The recent tedency in metro systems development (in middle-high rank towns) is going towards the so said "light metros", characterized by short rolling stock (and, conseguently, short stations) and high frequencies (normally between 3' and 65'').
These aspects allow a remarcable reduction of the construction costs, with transport capacity values similar to heavy systems, besides an easier insertion of the infrastructure in the urban contest, expecially when it has historical roots (like in many european towns).

APM
APM (automated people movers) must be mentioned among the light metros; these systems don't require any driver on board, an, generally, any fixed staff in the stations (except, in certain cases, for security and hospitality staff).
The system supervision is managed by a central control place, often located in the maintenance area.
The automation system is based on the classical ATO, ATP, ATS architecture managed automatically by a specifical software that works on the informations provided by a signalling system located along the track (track circuits, transponders); this allows the train detection on the track and the speed control in order to warrant the safety conditions and the respect of the timetables.
No fixed staff operates in the stations; safety and communications (on board and at the stops) are provided by video and audio systems that allow each moment to exchange infos between passengers and the central control place.
Platform doors are one of the peculiar elements of APM systems; they segregate passengers from the track, except for the train arrival moment, when these doors open in corrispondance with the train ones.


Links to builders sites:
ALSTOM
ANSALDO
BOMBARDIER
INTAMIN
SIEMENS