New electric trains with faster acceleration and more seats will cut end-to-end trips on ScotRail’s flagship route from an average of 51 to 47 minutes from 10 December.
However, only two services a day between Edinburgh Waverley and Glasgow Queen Street will be speeded up to 42 minutes, with some of the others following next year.
Other trains on the line will have to be switched from slower diesels first.
The move is the latest stage of the Edinburgh Glasgow Improvement Programme (Egip), which was due to have been completed in 2016.
Setbacks have included delays to electrifying the line and problems with the new Hitachi trains, such as their windscreens and brakes.
The trains have yet to be lengthened to eight carriages, and the redevelopment of Queen Street for longer platforms is not expected to be completed until 2020.
Under the original Egip scheme, journey times would have been cut to 37 minutes.
However, that was shelved six years ago when ministers slashed the project’s budget by one third to save money.
The new 42-minute services will stop three times en route.
By contrast, a new edition of The Railway Atlas Of Scotland by David Spaven refers to British Rail in 1971 promoting “the 43-minute journey”, which ran hourly, stopping only at Edinburgh Haymarket.
It also launched hourly services on the line – known as the “E&G” in the rail industry – that stopped twice and took 45 minutes.
BR’s “half-hourly high speed service” cut the journey time by 12 minutes, which it described in adverts as a “super-boon to all travellers”.
In his book, Spaven described this as “the apotheosis of speed” on the route.
Fellow author Ann Glen, in Transforming The Railways Of Central Scotland, said: “In their heyday, these trains gave travellers on the E&G the best service ever.”
However, journey times since lengthened as trains stopped more often and for longer, and now about half the passengers use intermediate stations. More trains also now share the line.
Spaven wrote: “Counter-intuitively, the inter-city journey has become progressively slower. The closure of Falkirk High tunnel for repairs led to a temporary direct service from Linlithgow/Polmont to Glasgow via Falkirk Grahamston, and users successfully lobbied for permanent calls to be made by E&G trains when normal service resumed, thereby slowing the end-to-end timing.
“A conscious decision was made to prioritise building up intermediate commuter business by adding extra calls to ‘express’ trains, at the expense of end-to-end journey times.
“With additional intermediate stops, the inter-city journey was stretched out to a standard 48–50 minutes.” This later became 49-51 minutes.
“So, when the full new electric service begins, the quickest Waverley–Queen Street journeys will be no faster than they were in 1980. This will be a very strange outcome for the modernisation of Scotland’s premier internal rail route.”
ScotRail wants to cut the fastest journeys to 39 minutes, while Transport Scotland said 37 minutes was still possible “subject to affordability and wider capacity choices”.
Scottish Conservatives transport spokesman Jamie Greene said: “Passengers will be dismayed to learn that only two trains a day will meet the 42-minute target we were promised. It’s unbelievably disappointing for a project that has ran £100m over budget and is delayed by 18 months.
“For years now, passengers have endured delays, disruptions, cancellations on the line in the hope journey times would be significantly cut. They’ll now be lucky to get a four minute reduction.
“Even British Rail, with all its documented problems, promised a 43-minute journey time nearly 50 years ago. That speaks volumes over the SNP’s ability to manage Scotland’s railways.”
A spokeswoman for the Scottish Government’s Transport Scotland agency said: “The comparison made to decades-old services is simplistic at best and all but ignores the significant benefits Egip brings to passengers and communities across the Central Belt.
“With access to more stops, more seats, more services and far more modern facilities, we are confident our investment in infrastructure and rolling stock will enable all users to enjoy a rail service between Edinburgh and Glasgow that far surpasses anything that has gone before.”