Holborn Tube station has been evacuated for a fire alert after passengers reported a "loud bang and smoke" on a platform.
A Transport for London spokesperson said west-bound Central Line trains were held while a "faulty train" was investigated.
The incident came on a morning of travel chaos in London, seeing one train partially derail outside Waterloo station and another hit the buffers at King's Cross.
Sarah Marshall filmed fellow commuters evacuating Holborn station as a platform filled with smoke.
She described a "loud bang, smoke and then an evacuation" at Holborn station on Twitter, adding: "There was smoke at one end of the station, a high pitched bang and a smell.
"Lots of people ran but others encouraged people to walk and situation was calm. I was towards back of crowd."
The London Fire Brigade said it has sent two fire engines and 10 firefighters to the scene.
“It was an overheated compressor on a train not a fire,” a spokesperson said.
“The Brigade was called at 9.09am and the incident was over by 9.58am.”
British Transport Police said officers helped to reopen the station after the evacuation "caused by a defective train".
The Central Line was temporarily suspended between Liverpool Street and Marble Arch and the incident caused severe delays on the Piccadilly Line.
Holborn station has since reopened and a full service resumed on all Underground lines.
It comes days after a small fire at nearby Oxford Circus station, where two people were taken to hospital to be treated for smoke inhalation on Friday.
London Underground said that blaze was caused by an electrical fault under a train carriage, which is under investigation.
Hours earlier, a train had partially derailed outside Waterloo station after crashing into a Network Rail engineering wagon.
South West Trains said there had been a "points failure" on the line, where engineering work is already causing major disruption on services to south-west England.
“There are no injuries to passengers or staff and a very limited service is running to and from Waterloo, which is expected for the rest of today," a statement added.
“The incident happened as the train was pulling away from its platform at Waterloo at 5.40am.
“An investigation to establish the cause of the incident is underway.”
Elsewhere, a train at King's Cross Station hit the buffers on platform nine in an incident being investigated by the Rail Accident Investigation Branch.
Network Rail said there was a “slow speed buffer collision” at around 6.20am and two people suffered minor injuries.
One passenger, Natasha Coella, described those on board going "flying" from one end of her carriage to the other after the train appeared to accelerate into barriers.
