These systems are typified in the UK by London Underground lines, Tyne and Wear Metro and Glasgow Underground. They normally operate along their own network of lines separate from the mainline railway, though interfacing is possible. Increasingly there is a move in some systems to automation of operating tasks to varying degrees. In urban centres they can often be found operating in underground tunnel networks.
Generally need relatively high passenger loads to justify the frequency of operation.
Peak speeds are not normally over 80kph; traction and braking performance being more important, vehicles sets often have a high proportion of motored axles.
Operation of freight across the routes can be possible but is often constrained by gauge compatibility, and path availability.