The choice of type of traction network (AC or DC) is influenced by the proximity of the generating plant. How come?
Direct current was first used commercially by Thomas Edison to power his newly invented light bulb. The limiting factor was it was not able to be sent very far away from the generating facility. Around 1950, alternating current took over as the new type of power for most commercial applications, chiefly because it was able to travel over much longer distances at lower current. DC power is analogous to a quarterback having to throw a 100 yard pass, what is known as a "Hail Mary" desperation, last-gasp attempt to score, to get the football to the endzone. AC power, on the other hand, would be similar to the multi-lateral type of football play where the ball is passed off to other players, diminishing the likelihood one player is going to be stopped. Alternating current involves the flow of electrons in different directions and therefore may be sent over longer distances without having to really crank up the voltage just to get it there.