The white bucket is stored in a Lidl bag with plastic wires coming out of it and is pictured on fire close to a seat.
Twitter users have reported seeing red wires coming out of the left hand side of the plastic bag.
The bag was on the District Line train at Parsons Green station with commuters reporting a 'fireball' ripping through the packed London Underground train.
Police have not given details about the suspected cause of the explosion but the incident comes with Britain on high alert after four terror attacks in 2017.
The official threat level currently stands at severe - meaning an attack is seen as "highly likely".
Eyewitnesses reported seeing several people hurt and "covered in blood" after a "flash and a bang" on the District Line Tube in West London.