explanation here :

"Propulsion

Propulsion is the force that drives the train forward. Maglev uses an electric linear motor to achieve propulsion. A normal electric rotary motor uses magnetism to create torque and spin an axle. It has a stationary piece, the stator, which surrounds a rotating piece, the rotor. The stator is used to generate a rotating magnetic field. This field induces a rotational force on the rotor, which causes it to spin. A linear motor is simply an unrolled version of this (see Figure 7). The stator is laid flat and the rotor rests above it. Instead of a rotating magnetic field, the stator generates a field that travels down its length. Similarly, instead of a rotating force, the rotor experiences a linear force that pulls it down the stator. Thus, an electric linear motor directly produces motion in a straight line. However, this motor can only produce a force while the rotor is above the stator. Once the rotor has reached the end, it stops moving. ECE_SHP_Figure07_v1_CornellWilson

Figure 7

Rotary motor versus linear motor. Source: Author, derived from Lee (2006).

When describing a linear motor, the standard is to use the term “primary” instead of “stator,” and “secondary” instead of “rotor.” In maglev trains, the secondary is attached to the bottom of the train cars, and the primary is in the guideway. So a magnetic field is sent down the guideway and it pulls the train along after it. In a way then, the entire length of a maglev track can be considered to be part of the train’s motor. The system that has been described so far is a Linear Induction Motor (LIM). It is so called because the magnetic field in the primary induces a magnetic field in the secondary. It is the interaction between the original field and the induced field that causes the secondary to be pulled along. However, in this configuration, the secondary always lags somewhat behind the moving field in the primary. This lag is a source of energy and speed loss. In a Linear Synchronous Motor (LSM), the lag is removed by attaching permanent magnets to the secondary. Because the secondary is now producing its own stationary magnetic field, it travels down the primary in sync with the moving field—hence the name for this variant of motor (Gieras, 2011). Because LSMs are faster and more efficient, they are the motor of choice in high-speed maglev trains (Lee, 2006)."
https://sites.tufts.edu/eeseniordesignhandbook/2015/maglev-m...