The end of coal trains to the Fife plant would free up tracks so passengers could travel on the Forth-side line between Alloa and Dunfermline for the first time for 85 years.
The scheme could include re-opening a station at the historic village of Culross to improve access for tourists.
Trains supplying Longannet used the route until the £85 million Stirling-Alloa-Kincardine line re-opened in 2008.
Passenger trains run on the new line only as far east as Alloa, but when coal trains cease, there is the potential for ScotRail services to continue via Kincardine to Dunfermline and over the Forth Bridge to Edinburgh. This could build on the success of the Stirling-Alloa section, which has carried three times as many passengers as expected.
The Kincardine-Dunfermline line closed to passengers in 1930 but is used by occasional steam charter trains.
Longannet operator ScottishPower told MSPs last week the station could close within a year unless it wins a short-term contract. However, the plant faces other threats to its survival, such as high transmission charges, and may have a limited future burning coal.
Public transport lobby group Transform Scotland said the chance to reinvigorate the rail line should be seized.
Paul Tetlaw, its rail campaigner, said: “It would be the perfect opportunity to build on the huge investment of constructing the route through Alloa to Kincardine, by creating a passenger service.
“It would open up the north bank of the Forth to all sorts of new travel possibilities, such as to the delightful village of Culross, and also provide access to both Glasgow and Edinburgh for work and leisure.”
Dunfermline Labour MSP Cara Hilton said improved transport would be key following any closure of Longannet, which she is fighting to save.
She said: “It will be absolutely vital for Fife Council and the Scottish Government to do all they can to build up the resilience of the area. Much better transport links, in the form of a reopening of the Kincardine train line, would boost long-term regeneration, opening up significant employment opportunities.”
A Network Rail spokesman said: “The Stirling-Alloa-Kincardine line was constructed principally to carry freight and there are currently no plans for the extension of passenger services.
“If freight use on the line were to change in the near future, we could review how the line is used. Any proposals to extend passenger services would need to meet a clear need and be supported a business case and funding package.”
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