Hybrids use diesel power where there were no overhead electriccables so could be used across the network without having to change locomotives or crews.
For a metropolis twice the size of another, the length of electriccables, number of gas stations and other bits of infrastructure decrease by about 15% per inhabitant.
The Superdome's power company took the blame Friday for the Super Bowl blackout, saying the cause was a faulty device that had been installed in its switching gear to prevent a failure of electriccables leading to the stadium.
New York City, electric power is transmitted by high voltage cables encased in steel pipelines to electric substations where the power is converted to low voltage feeders supplying about 55 distribution networks.
Although they have not been found to cause any harm, the cables on conventional pylons, which transmit a three-phase alternating current, generate a strong electric field and a continuous buzz of low-frequency radio waves.