.. and incompatible with existing train systems in most regions. In many systems you currently have a central hub which is overhead wired. If you then have a

.. and incompatible with existing train systems in most regions. In many systems you currently have a central hub which is overhead wired. If you then have a unwired branch line installing a third-rail-system there would require vehicles which can use both. Such systems exist, like the underground in Nuremberg, Germany, which operates with a third rail on tracks, but in the maintenance area has overhead power to make work safer. However that increases cost by requiring custom vehicles etc.