Electric circuits blowing in a rail tunnel injured an engineer in the Kilburn High Road building above.
London Fire Brigade said firefighters were called to reports of a smell of chemicals at a shop at 4pm on Tuesday (May 25).
The brigade said crews found no elevated readings of chemicals but that there had been some electrical arcing.
Police said an engineer working at the premises was injured. He was taken to hospital by ambulance.
Man-in-hospital-after--possible-explosion--at-shop-toilet-in
One witness said on Twitter: "A toilet blew up in Cooke's next door to the Old Bell pub. We had to stay inside until it was safe."
They told The Times: "I heard a loud bang and thought it was an explosion or train crash."
A Network Rail spokesperson said: “We’re investigating an incident with the overhead electric wires which power trains on the West Coast main line in Kilburn.
"The electric circuit blew in a tunnel underneath Kilburn High Road at 5.30pm on Wednesday May 25. As a precaution one shop above the tunnel is closed today. We’re supporting its owner while we determine the cause of the overhead line fault."
Police said the incident is not being treated as suspicious.
The Times has been unable to reach Cooke's Amusements for comment.
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