A woman, who has requested to be known only as Karen, was travelling on the Belgrave-bound train about 6.30pm on May 18 when the man sitting opposite her snatched her handbag.

Karen was struggling with the man, when the other commuter stepped in.

She said the good Samaritan "very politely" asked the would-be thief to return the handbag.

The bag-snatcher obliged and dropped the handbag, before moving to the other end of the carriage.

Police are keen to speak to the man who intervened and are appealing for him to come forward.

"I would highly applaud the good Samaritan in this case," said Detective Senior Constable Tom Holmes.

"His actions were very brave."

Police want to speak to the good Samaritan to obtain his version of the incident, Senior Constable Holmes said.

He said police were also examining CCTV footage of the incident.

Karen said she had since become wary of catching public transport, but would not let the experience change her travel habits.

"I was terrified," she said.

"I am nervous now around catching trains...[but] I enjoy my train rides...he won't take that that away from me.

"I don't want people to be scared of public transport."

Karen's handbag had contained precious family heirlooms, including her grandmother's wedding rings and her grandfather's war service medals.

Her own wedding and engagement rings were also in the bag at the time.

"I would have lost things that I could never have replaced," she said.

Earlier this week, the actions of Jason Cias were applauded after he went to the defence of three Muslim women being racially abused on a Craigieburn-bound train from Melbourne Central.

Anyone with information is asked to contact Crime Stoppers on 1800 333 000.