British Transport Police (BTP) said a man was assaulted on the Central Line tube on February 1, at around 7.25am.
It came after the man had spotted three youths using laughing gas and abusing other passengers before he intervened, officers said.
Police said a group of youths assaulted him, and he was hit on the head with a medium-sized gas canister.
The youths then got off the train and ran off, BTP added, and the man was hospitalised.
A poster had appeared at Ealing Broadway station appealing for witnesses to the incident.
Anyone with any information can call BTP on 0800 40 50 40 or text 61016 quoting 86 of 01/02/2017.
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