The health service employee was also punched repeatedly during the attack, which occurred after he pointed out that it was a breach of coronavirus restrictions for the trio – two men and a woman – not to be in face coverings.
One of the group struck the victim several times before throwing him off the train and onto the platform at High Street Kensington station.
He also suffered a chipped jaw.
The three stayed on board as the train departed.
The incident – which occurred at 10pm on Saturday 3 October – only came to light on Tuesday night after British Transport Police released pictures of a man they say they want to speak to in connection with the attack.
He is white with short blond hair and wearing a suit. He was described as carrying an umbrella.
Figures released on Monday show almost 500 people have been fined in London for not wearing face coverings on public transport.
Enforcement officers had also stopped about 113,000 people from getting on public transport services until they put on a face mask, while 1,800 were removed from services.
Anyone who recognises the man in relation to the High Street Kensington attack should contact on 0800 40 50 40.
Watch: How to remove a face covering correctly