RAIL operator Metro has ordered a risk assessment of the use of armed protective services officers at railway stations, amid growing opposition to the Baillieu government's plan.

But Police Minister Peter Ryan rejected concerns of police and unions about the potential risks to the public and the officers and vowed to press ahead whatever the findings of the review.

Under the government's plan, 940 protective services officers (PSO) armed with semi-automatic firearms, capsicum spray and batons will be deployed at railway stations from 6pm until close.

A Metro spokesman confirmed it would undertake the risk assessment and said it would do so with any change to work practices. He said the review was at a ''scoping'' stage.

Rail, Tram and Bus Union state secretary Trevor Dobbyn said Metro - which has previously raised concerns about the proposal - had agreed to the union's request for an assessment and said the operator would look to ''identify and mitigate'' the risks involved.