More than 500 peak hour commuters were evacuated from Sydney's Town Hall station on Monday morning.

A smoke cloud from a motor room at the station triggered an automatic alert, forcing the evacuation of the CBD transport hub.

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NSW Fire and Rescue were called to the station at 9.33am as an overheating bearing in the motor room caused smoke to enter the station.

The station was quickly evacuated by emergency authorities.

However, Sydney Trains have confirmed the station has reopened and services have returned to normal after a brief period in which trains did not stop at Town Hall.

No firefighting reinforcements were required, Fire and Rescue said.