Diggers will start earthworks in the next two weeks for a second Penrose railway station, needed to re-open the Onehunga branch line to passenger trains.

That follows Auckland Regional Transport Authority and KiwiRail confirmation of sites for the three main stations for the line, which is to re-open in the first half of next year.

The other two stations will be between Mays Rd and Captain Springs Rd in Te Papapa, and at a site off the lower end of Onehunga Mall, which the Auckland Regional Council has bought for about $8 million.

KiwiRail will build a short spur to platforms on the site, to keep the main line clear for freight trains to and from the Onehunga waterfront and airport passenger rail services.

A fourth station near Mt Smart Stadium for special-event trains remains a possibility.

The regional transport authority is considering running trains every half an hour at peak times and every hour between peaks and at weekends, although it has yet to definite decisions on the frequency of services.

Spokeswoman Sharon Hunter said the authority would keep in mind a request from the Campaign for Better Transport for half-hour services at all times.

The campaign, which collected 8000 signatures for a petition to reopen the line, says half-hour services are needed to draw shoppers to Onehunga Mall and the nearby Dress-Smart centre and to cater for SuperGold card holders entitled to free public transport after 9am each day.

Maungakiekie Community Board chairwoman Bridget Graham said of the transport authority's proposal for hourly off-peak services that "we have got to start somewhere".

But she believed the resurrection of passenger trains, which stopped running to and from Onehunga in 1973 after a century of operations, would prove more popular than the transport planners envisaged.

Community interest was such that more than 100 people crammed into an Onehunga hall last week to study station location plans, and Ms Graham said she had received calls from people asking when the trains would start running as they were interested in buying homes in the area.

"It's very good news for Onehunga," she said of the project.