In his letter to the first minister, which was first reported by BusinessLive, Thomas said: “The near three-year delay this project has endured is as inexplicable as it is indefensible.
“It is an affront to local decision-making; it has left a poor and marginalised community in limbo; it flies in the face of ambitions to grow the economy, and it portrays Wales in an extremely unfavourable light to would-be investors.
“You have a historic opportunity to right this wrong and to set a new direction which is bold and ambitious. I would urge you to seize the moment.”
Thomas urged Morgan to announce her decision before the end of January and called for the project to be “front and centre” of a planned investment summit being organised by the Welsh government later this year.
Cardiff Parkway Developments, which is overseeing the project, hopes to build a four platform railway station and a business park.
Its original agreement with Cardiff council means the railway station must be built during the first phase of construction.
However, the Welsh government’s planning team has sought further reassurances that this remains the case.
The Welsh government has a minority stake in the organisation, with Investec and the Roberts family.
A government spokesperson said: “Consultation is ongoing with organisations and individuals about the development.
“A decision will be made following consideration of the representations received.”
In a statement to BBC Wales the Roberts family said it expected the latest planning requirement to be “straightforward”, with papers due to be filed by 15 January.
“We hope this means we’re near the end of the long call-in process, and can move on to the next steps of preparing for delivery.”
