A brief history and theory behind AC, DC and maglev trains and subways

This paper gives a brief history of the last 125 years of electrically propelled locomotives. It discusses the uses of DC powered motors as the mainstay of locomotive power for the first 80 years. Also talks about the technical advances that allowed for the industry to advance to a more efficient AC motor with the invention of such semiconductor devices as the diode and GTO's. Lastly it discusses the ability to magnetically levitate and propel a locomotive with maglev technology.