Part of the main Edinburgh-Glasgow route will be closed from 13 June to 27 July, which coincides with tens of thousands of fans heading for the Open golf at St Andrews.
The Winchburgh tunnel in West Lothian is being closed as part of the £750 million electrification of the line to enable faster and longer trains to run.
During the work, journeys are expected to take up to 50 per cent longer than normal because trains will be diverted via Dalmeny, beside the Forth Bridge, then have to reverse on to another line.
Some trains may have to be cancelled, reducing the current 15-minute frequency service between Scotland’s two largest cities. Services between Dunblane, Alloa and Edinburgh are also likely to be hit.
At the last Open in Scotland in 2013, ScotRail carried 25,000 people to the championship at Muirfield in East Lothian on 100 special trains.
Rail campaigners said a new section of track – the “Almond Chord”, enabling trains to run via Dalmeny without reversing – should have been built first.
Roderick McDougall, vice-chair of independent campaigners Railfuture (Scotland), said: “We deprecate [Scottish Government agency] Transport Scotland’s insistence in closing the Winchburgh tunnel, causing major disruption to passengers both on the main Glasgow to Edinburgh route and the Stirling to Edinburgh, before constructing the Almond Chord, particularly as they have admitted that it is still intended to build it.
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“It does not seem feasible for all six trains per hour in each direction to reverse at Dalmeny, so I assume some will have to be curtailed, possibly reducing the Glasgow-Edinburgh trains to half-hourly.”
David Sidebottom, passenger director of watchdog Passenger Focus said: “Most passengers will benefit in the long term from the electrification. However, passengers won’t be happy about the disruption this project will cause. The rail industry will need to work hard to give passengers plenty of notice of changes to timetables and be on hand to provide them with the necessary help and information.”
The 400-yard-long tunnel, built in 1842, is being enlarged to fit in overhead wires. Network Rail, which is in charge of the project, was unable to say what disruption would be caused.
However, during previous tunnel closures due to flooding, 50 minute Edinburgh-Glasgow journeys have increased to around one hour 15 minutes.
The rail firm said it would upgrade signalling to accommodate more trains running between Dalmeny and Edinburgh during the work.
Its spokesman said: “To minimise disruption, trains will be routed via Dalmeny and Network Rail will be carrying out signalling improvements in advance to increase capacity and reliability between Haymarket and Inverkeithing.”
The Scottish Government said Abellio, which has won a ten-year contract to run ScotRail from April, was already planning for the work.
A spokeswoman said: “Transport Scotland continues to work closely with both Network Rail and ScotRail to minimise the duration of the closure.
“We expect ScotRail to develop a timetable that best serves the needs of customers and to accommodate additional demand generated by the Open Championship at St Andrews.
“Abellio is aware of the proposed blockade works required and in its bid noted the importance of planning for the Open.”
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