The Herald has learned that executives at Network Rail, the quasi-public concern that already runs rail infrastructure, have discussed operating passenger services as well.

A senior company official has held informal talks on the proposal with one of the Labour Party's most influential policy makers.

The move would see a not-for-profit trains division of Network Rail compete with commercial firms for the right to operate services north of the border.

Any change would take place when the ScotRail franchise, currently held by First Group, expires in 2011.

Effectively, a Network Rail operating company would have all the advantages of the publicly-owned CalMac, which was the sole bidder for ferry routes.

The Labour Party this week included a pledge to investigate the issue in its manifesto.

It stated: "The case for running the Scottish franchise on a not-for-profit basis needs to be fully examined as part of the preparation for the next franchise."

The Scottish Parliament does not have the power to renationalise the railways. It can, however, change the terms of its franchise agreement to remove the opportunity for profit.

Network Rail officials, despite officially denying any interest, are understood to support such a policy.

It is believed an understanding was reached during the Scottish Labour Party conference in Oban last November in a private discussion between a senior executive of Network Rail and a Labour politician involved in developing transport policy. At the conference the policy of "examining" a not-for-profit model was later agreed.

Any Network Rail operating division would be free to tender for other franchises in other parts of the UK.

Theoretically, a private firm could still operate the franchise for a flat fee. The agreement could stipulate that any surplus generated must be invested in services. First ScotRail currently receives two-thirds of its income from the Scottish Executive and collects the remainder in commercial operations and fares.

The attractiveness of Network Rail is thought to be that it already runs the infrastructure on a not-for-profit basis and could re-integrate the majority of the rail network under one organisation.

Unions support the proposals, which will be debated at the STUC conference next week. Gerry Doherty, general secretary of the Transport Salaried Staffs' Association, said: "What we would be looking at would be vertical integration of the railways.

"We are calling for the train operating companies to be run on a not-for-profit basis and the next step would be for Network Rail to be the train operator."

Independent rail experts were sceptical about a single firm running track, stations and trains but said the network north of the border was more suited to such a move than any other region.

Mike Crowhurst, the chairman of Railfuture, said: "Scotland is more self-contained than other areas but you still have the freight operators and cross-border services, Virgin and GNER to think of. I am not sure it could work in isolation. What it amounts to is a form of renationalisation."