Network Rail has responded to comments from Birmingham's chief planning officer, Clive Dutton, who said the designs, by John McAslan + Partners, lacked the 'wow factor'.
A Network Rail spokesman said: 'The designs, which the council are commenting on, were never going to be our final designs. They were merely to give people an idea of how it might look - they are outline designs.
'It's an ongoing process, and we will be submitting more detailed designs when we go in for planning at the end of the year.'
The response follows comments from Dutton, who spoke during a visit to the site last week.
He said: 'A world-class city deserves a world-class station. We should be negotiating absolutely excellent design here.
'This is a highly significant proposal in terms of our international city aspirations. It is so important to get it right.'
The scheme will be going in for planning when funding is secured, which the developer hopes will be by the end of the year.