The introduction of a half-hourly rail service between Norwich and Cambridge cannot go ahead even if a junction improvement proposal at Ely is approved, it has been learnt.

The single-track Trowse railway bridge just outside Norwich station is a major impediment to the improvement of train services in Norfolk, it emerged yesterday.

Network Rail surprised political leaders from the county at a meeting in Westminster yesterday by stating that the pinchpoint would stop the introduction of a half-hourly service between Norwich and Cambridge even if it invests in a junction improvement at Ely that would open up such a service between King's Lynn and Cambridge.

The Trowse bridge carries all incoming and outgoing trains on the lines from Norwich to Ipswich and Cambridge, and it might not be possible to overcome the capacity constraint by doubling the track. A new bridge might have to be constructed, and 'we were told that would cost an awful lot of money', said Graham Plant, the transport and planning portfolio holder on Norfolk county council.

Norfolk and Suffolk county councils are to work together on producing a masterplan for radically improving the 'Cinderella' rail services in the region over the next 25 years.

The immediate top priority is to persuade Network Rail to invest in a �10m improvement scheme for the junction at Ely, which is a key cross-over for north-south and east-west rail services in East Anglia.

The first beneficiary would be the service between Lynn and Cambridge, which could be improved from hourly to half-hourly. The freight service between Felixstowe and Nuneaton might also benefit straightaway. But the Norwich-Cambridge service would have to remain hourly because of the Trowse bridge blockage. There are also hopes that the Ely scheme would allow the service between Ipswich and Peterborough to be raised from two-hourly to hourly.

Norfolk SW MP Elizabeth Truss is to have a meeting with treasury minister Lord Sassoon to try to get his department to back the business case for the Ely scheme and add to pressure on Network Rail to provide the funding for it.

But it seems the next priority for the new Norfolk-Suffolk rail strategy team will be trying to resolve the Trowse bottleneck. Mr Plant said he had not known until the meeting that it would stop improvements on the Norwich-Cambridge line.

Following the meeting, a Network Rail spokesperson said: 'We looked into whether two trains an hour could run between King's Lynn and Cambridge and concluded this was not possible on the current infrastructure. However, there could be a case for works to be carried out which would allow this to happen. We have done some initial work to look into the business case for improvements to the line and are now working with local councils to get a more complete picture of the case for investment.'

On Trowse bridge, he said that this was mentioned as part of a wider discussion at the end of the meeting about other potential improvements to the railway across the region. 'One of the MPs asked about ways to increase services between Cambridge and Norwich. In answering this, it was explained that a new bridge would probably be needed to provide a half-hourly service between Cambridge and Norwich – but no detailed study has been carried out. This is not what the meeting was about and was separate to the discussion about the King's Lynn line', he continued.

The admission by Network Rail about Trowse led to criticism of it by other representatives at the meeting. Two told the EDP that it was it a long-standing capacity problem that should have been identified and rectified years ago.

The Norfolk and Suffolk county councils will develop a formal structure and strategy to secure better rail services for the region as part of a 'joined-up thinking' approach to promote economic growth and make it easier to absorb more housing. Mr Plant said the councils had never done this before, and this was reflected in the region's poor rail infrastructure.

'That has all changed', he emphasised. The government was looking to improve infrastructure to raise growth, and local political and economic leaders wanting better services would be 'knocking on an open door' as long as they could show their case was viable.

Ms Truss chaired yesterday's meeting. It had been 'extremely positive' she said, 'and I was delighted to see the commitment by all the various parties. We have focused and identified the main points that need to be presented to the treasury and the department for transport to secure the �10m required for the Ely upgrade. I will be writing to the commercial secretary to the treasury, Lord Sassoon, as soon as possible to ensure that he is fully aware of the benefits of investing in the Ely North junction.'

Andy Tyler, secretary of the Fen Line users Association, said: 'It's absolutely vitally important, it's a growing successful line between King's Lynn, Ely and Cambridge. We have been calling for a long period for a half-hourly service. We're delighted that local MPs are pressing for it, we support that very much.'