Network Rail has launched the process of selling the 137km of Valley Lines routes in south Wales to the Welsh Government.
The proposed deal, which has been notified to key people in the railway industry, is the first sale of its kind since Railtrack was created in 1994.
When it has gone through, the Valley Lines network, including Cardiff Queen Street station, will be outside the control or responsibility of Network Rail, in the same way that Transport for London already owns and maintains some 400km of railways.
A one-month consultation started on Friday (22 February), and the proposed sale date is 22 September, with completion of the handover due to follow in March next year.
The transaction is expected to require the approval of the Office of Rail and Road.
The Valleys network is to be modernised and electrified to become a core element in the South Wales Metro. It will be controlled from a separate signalling centre at Taffs Well, where a new depot is also being built.
The lines are being sold on a freehold basis, including buildings and operational assets.
The routes are Cardiff Bay via Cardiff Queen Street to Rhymney and Coryton plus the routes from Cardiff Queen Street North Junction to Merthyr Tydfil, Aberdare and Treherbert, plus the freight lines to Hirwaun and Cwmbargoed.
After the handover it would still be possible for services to run through from Network Rail infrastructure, such as local services from Cardiff Central.
The sale comes after repeated calls for Network Rail north of the border to be devolved to the Scottish Government, and it has also come before Keith Williams has had the time to complete his DfT-sponsored review into the future structure, ownership and operation of rail services in Britain.