Why is a 3 phase induction motor is used in electric trains?

AC induction motors are not always used in trains as traction engines, however they are a popular choice. AC induction motors are very simple and reliable and aside from bearing wear they have no parts that wear out. This is because the power used by the rotating parts is supplied by the magnetism like in a transformer and so there are no slipping brushes.

Most train systems actually use a DC supply, then a device called an inverter is used to convert the DC to variable frequency to drive the AC motor and control its speed. When this is done the AC induction motor is a very reliable and flexible engine and even allows energy recovery when the train is breaking.