There is quite a variety of weir bus mutations out there and so far none of the systems fared particularly well.
The original trolley bus system is nothing but a bus with an electric motor and a 2 wire overhead line. This combines the disadvantages of a rail system (not much ability to evade obstacles, maintenance cost of the overhead wires) with those of a Diesel/gas bus (less comfortable ride for the passengers, equipment and operators, damage to the street surface due to the heavy vehicles, large bend radii).
The there have been several tries to improve those systems by keeping th bus on track, either by adding a center rail keeping the bus in line or by uing "virtual rails", an electronic positioning system. The first system failed catastrophically in several French cities, with frequent derailments and horribly bumpy rides. It simply did not work. The latter system never made it past the project stages I think.
Urban transit systems usually operate on 500-900VDC with very few exceptions (some German streetcars run off the 15kV 16.7Hz railway supply and can run on regular railway lines too, in Vienna loooong ago we had a 750V 16 2/3 Hz system, the cars could operate on DC too, inside Vienna they ran off 600VDC, outside (25km to the nearby town of Baden) on AC).
