In the common definition these systems have a completely segregated track, mostly underground.
The traditional metros can reach high capacities (> 10000 pphpd) thanks to the segregated tracks and to the signalling systems (with related high frequencies); the heavy impact of the construction has been reduced by the adoption of bored tunnel machines (TBM), instead of the cut&cover method (mostly used only for the stations).
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 remarkable reduction of the construction costs, with similar values of transport capacity in comparison with heavy systems, besides an easier insertion of the infrastructure in the urban contest, expecially when it has historical roots (like in many european towns).
Links to builders sites: