Passengers were evacuated from Holborn station shortly after 9am on Tuesday after a smoke alert on a Central Line train.
London Fire Brigade says two fire engines and 10 firefighters examined the area and that the incident was caused by "an overheated compressor on a train, not a fire".
The British Transport Police say they are investigating but indicated on Twitter the evacuation was caused by "a defective train".
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The station has now reopened and service has resumed with severe delays.
It comes amid a day of travel chaos for British commuters, as a train hit buffers at King's Cross, and a partial derailment at Waterloo.
It happened less than a week after Oxford Circus tube station was closed because of smoke from an electrical fault.
