Regional rail lines connect towns and cities with one another,
and larger cities have networks of commuter rail lines running from outlying towns and
suburbs to the city center. These types of trains come in various configurations and can
be powered by diesel or electricity. Electric propulsion is generally more
energy-efficient and less polluting than diesel, especially when the electricity is
produced by modern power stations or renewable sources. Further energy savings can be
achieved by utilizing regenerative braking, in which energy that is normally dissipated as
heat when a vehicle brakes is fed back into the power grid. The infrastructure required
for electrification is costly, and is only justified on heavily-traveled corridors.
