Half-hourly train services from the borough to the capital were scheduled to be introduced in two years’ time, following a lengthy campaign by rail user groups and politicians.
But that depends on vital expansion work at the Ely north junction, which officials say they are aiming to complete by early 2019, being completed.
And representatives of the Fen Line Users Association, which represents passengers using the route from Lynn to Cambridge, have warned passenger numbers have outgrown current infrastructure.
In a joint letter, group chairman Colin Sampson and secretary Andy Tyler said they welcomed the government’s commitment to the line.
But the pair, who are also Conservative and Labour councillors respectively, added: “Our campaigning call is for more and longer trains and we have been working with politicians of all main parties for some time to press that message home.
“However, many of the changes we need are not coming quickly enough.”
The work, which is expected to cost around £25 million to complete, has been included in Network Rail’s programme of works for the 2014-18 spending period.
And, under the terms of its franchise agreement, Great Northern, the company which took over the operation of passenger services from Lynn, Watlington and Downham to London Kings Cross last September, is due to introduce half-hourly trains in May 2017 if the upgrade is completed.
The key sticking point is the need to resolve safety issues at three level crossings immediately to the north of the junction.
A Network Rail spokesman said yesterday: “We are committed to upgrading Ely North Junction as part of our investment programme, from 2014-2019.
“This planned increase in capacity means we have to consider any impact on level crossings in the area.
“Our aspiration is to deliver both projects around the same time to minimise any potential disruption.
“Our aim is to deliver the upgrade by 2019 and we are currently working on the plans for the upgrade.
But, during a visit to the station this week, environment secretary and Conservative election candidate Elizabeth Truss urged Network Rail to ensure the work is done sooner rather than later.
The comments followed a meeting she had with the firm last month.
She said: “There are additional challenges with the upgrade in relation to the road crossing.
“Network Rail are working with the various authorities to find a suitable solution.
“However, I am very keen to ensure this remains on track and that there is no slippage in the timetable of works.
“Working with key partners, every support will be given to ensure this can progress as soon as possible.
“Ely North is such a key junction for passenger and freight transport that it is vital any delays are kept to the absolute minimum.”
The project would dual the tracks to the north of the station, enabling more passenger services to run on the Lynn line and the cross-country route to Norwich, as well as an expected increase in the number of freight trains travelling between Felixstowe and the Midlands.