Longer trains will carry passengers to and from West Norfolk, but are unlikely to do so until at least the end of next year, rail chiefs say.
The admission came as industry bosses met political and business leaders in Downham to discuss the ongoing work to improve train services across the region.
And a borough MP has warned the industry must deliver on its pledge to tackle what he called “unacceptable” levels of overcrowding on the route.
At present, most trains running between Lynn and Cambridge have four coaches and are coupled to another four coach unit at Cambridge before continuing to London.
The current aspiration is for eight-carriage services to run all the way to Lynn, with work both to extend platforms at smaller stations and selectively lock carriages from which passengers cannot step onto a platform.
But, although industry officials had previously expressed an aim to introduce longer trains during 2016, Friday’s rail summit in Downham was told they now expect to start running eight-carriage units by December 2018.
Chris Rowley, of Network Rail, said: “We know overcrowding at peak times is a big issue and something that’s been flagged up for years.
“The bottom line is the development is proceeding well. We’re going for permission next week to progress to detailed design work.”
“From a standing start last year when we had to relay bad news, we still expect to be able to deliver an eight-car railway by the end of this control period (2018-19).”
But North West Norfolk MP Sir Henry Bellingham told them the current overcrowding, particularly at peak times, was a “joke” and sent the wrong signals to companies considering investing in the area.
Speaking after the meeting, he said: “We’ve got to absolutely hold their feet to the fire.
“That is going to solve the problem of massive congestion. At the moment it’s almost sort of Third World standards.
“It is completely unacceptable and the signal that’s sending to business people, to entrepreneurs, investors, is incredibly bad.”
He also pledged to keep up the pressure for a half-hourly service to and from Lynn after representatives of Govia Thameslink, the parent company of Great Northern, West Norfolk’s main train operator, indicated they would not expand the service until work to upgrade the Ely north junction had been completed.
Although the company had committed to running two trains an hour from later this year, it said that was dependent on the junction being improved.
But Sir Henry said: “They’re actually going back on their original commitment, which I find disappointing.”