It aims to rebalance spending between rail and road, but the tunnel alone would cost as much as the dualling of the A9 and A96 combined, which are themselves some of the biggest projects in Scotland’s history.
The Greens said the project, estimated to cost £4-6bn, would cut journey times by 25 minutes to bring Dundee within an hour of Edinburgh.
They said it would also ease a bottleneck around Haymarket Station in the west of Edinburgh “that affects the whole network” by creating a new route into the capital.
The tunnel would be built from between Kirkcaldy and Seafield to Leith, where there would be a station underground stretching from near the waterfront to the foot of Leith Walk.
That would be connected to Waverley Station via the former Abbeyhill loop line.
The Greens' Rail For All policy briefing stated: “A Forth tunnel would be a game-changer, vastly reducing travel times, improving punctuality, and releasing enormous capacity over the Forth Bridge that could be used for extra services to, for example, Dunfermline, West Fife and Alloa.”
Deltix Transport Consulting, which was commissioned by the Greens, said in the report: “It should be noted that there has been a long history of boring under the Forth for coal mining, and railway tunnels much longer than this have become relatively common in mainland Europe.
"The very recently opened Ceneri Base tunnel in Switzerland, which at 9.6 miles is virtually the same length, had a quoted cost of €3.6bn (£3.2bn).
Deltix associate David Prescott, who co-wrote the report with David Spaven, said: “At a stroke, the Forth tunnel can transform the geography of Scotland, just as the Forth Bridge did 130 years ago and the Forth Road Bridge nearly 60 years ago.
"It would substantially cut the distance between Edinburgh and east Fife, Dundee, Aberdeen, Perth and Inverness, whilst also putting Leith at the heart of the Scottish rail network.
"The Forth tunnel would give this generation of Scottish engineers the opportunity to continue the strong traditions of their illustrious predecessors - creating transformational infrastructure for the nation.”
The Greens’ proposals also include creating another Glasgow city centre rail terminal by building above the underground Argyle Street station at the west end of the St Enoch shopping centre.
The Dunfermline-Alloa route would be re-opened, lines across the Highlands and to Stranraer upgraded, and where possible, towns of 5,000 people or more connected to the network.
“Tram trains” which could run on streets as well as the rail network could operate on a re-opened cross-Clyde link from the expanded Argyle Street Station, as well as in Aberdeenshire on restored lines serving towns such as Ellon and Banchory.
The report said that for every £1bn invested, there would be a £1.4bn boost to economic activity, along with the creation of 14,000 jobs.
Greens transport spokesman John Finnie said: "The Scottish Greens are proposing the biggest rail investment programme Scotland has ever seen.
"Our fully-costed £22bn plan would transform Scotland's railway, building a modern, zero-carbon network that is affordable and accessible to all.
"The investment would also be a central component of Scotland's green recovery from the Covid-19 crisis, creating thousands of quality, unionised jobs whilst delivering the infrastructure so necessary to tackle the climate emergency."
However, Scotland’s leading transport academic cast doubt on the tunnel plan.
Professor Iain Docherty, Dean of the Institute for Advanced Studies at the University of Stirling, said: “I don’t agree with the priorities for investment set out in this report, but it is an important and welcome contribution to the debate about how we should rebuild as we emerge from the pandemic, and shift focus to decarbonisation.”
The Scottish Government said a Forth rail tunnel had been considered in 2007 as part of a study that led to the building of the Queensferry Crossing.
However, it said the report concluded the most cost-effective way of increasing cross-Forth rail capacity was by lengthening trains and timetable improvements. to maximise the number able to use the Forth Bridge.
Its spokesperson said: “Aspirational projects, such as the Firth of Forth tunnel, would be subject to the same scrutiny from the earliest stages to ensure they merited any serious consideration.”
They said ministers were also “willing to consider proposals for new stations, or reopening existing ones, that arise from a positive transport appraisal”.
Major schemes planned for the next 20 years are due to be unveiled next month with the publication of the Scottish Government’s Strategic Transport Projects Review.
The Greens are also pushing for the rail network to be publicly operated.
Their report said: "Re-integrating ScotRail and Network Rail (Scotland) into one publicly-owned company with oversight by Scottish ministers would provide a much more efficient structure, cutting costs by reducing overlapping work, speeding up decision making and project design, and removing the cost of compensation paid when lines are shut for engineering works.
"Effectively, we recommend a return to the very successful ScotRail that existed before privatisation."
The Scottish Government spokesperson said: “We have stated for some time now that the franchising model is no longer fit for purpose and an integrated, public sector controlled railway, fully accountable to Scottish ministers and Parliament will best serve Scotland’s economy and its communities.
"Repeated calls on UK ministers to give Scotland the powers needed to secure the best future for Scotland’s railway network, and to remove the absurdities and anomalies of the current system, have been denied.”