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: