A review of the West Midlands railways is under way, by the backers of a £500m revamp for Birmingham New Street.
Birmingham Gateway backers say rail congestion is being tackled
The Birmingham Gateway redevelopment was announced in February and aims to give the ageing station a new look.
But there have been complaints that it does not mean New Street - already facing congestion leading to delays - could handle any more trains.
Network Rail is expected to deliver its review of rail capacity in Birmingham and the West Midlands by mid-June.
It is looking at long-term rail growth and the challenges this would bring to the network.
Passengers arriving and leaving New Street station often face delays with trains held up waiting for a platform because of congestion.
The Gateway project is mainly concerned with the appearance of the station, plus its shopping and facilities above the platforms.
Backers of the Gateway project have said their project would be a "major investment" in the public transport network but involve "minimum disruption" for passengers.
The city's 1960 station could be rebuilt within a few years
They said that confusion and distraction about the development risks Birmingham's rail transport network going unchanged for the foreseeable future.
The backers - Network Rail, Birmingham City Council, Advantage West Midlands and Centro - have issued a joint statement to support their scheme.
They said: "Addressing rail capacity in the West Midlands, today and for the future, is clearly important to the leaders and the people of the region.
"We already know that Birmingham New Street can cope with significant future growth through measures such as the lengthening of trains.
"However, a serious review of rail growth in the much longer term is the only way to establish the exact nature of the challenges that we face, and how these challenges can best be met.
"Such a review is presently being undertaken by Network Rail and represents a sensible piece of due diligence for a major piece of public investment."