Discussions over expanding the MetroWest line 30 miles beyond Yate to Gloucester will conclude by the summer.
Proposals for the extension received a boost when the government announced it would back the idea last year.
Director of infrastructure for West of England combined authority David Carter said Weca was working with Network Rail on the necessary infrastructure needed.
The MetroWest project aims to improve suburban train services into Bristol.
Mr Carter told councillors Weca asked for MetroWest 1 to be extended to Westbury and MetroWest 2 to be extended to Gloucester in autumn, and it was up to the Department for Transport to negotiate with Great Western Railway, to determine whether they will or will not accept that.
He said: “We have to make sure we keep, in both MetroWest 1 and 2, the cost of the turnback at Bath and the cost of the turnback at Yate until such time as the department confirms Gloucester and Westbury are in the franchise, and we won’t know that until June or July.”
Mr Carter added Weca was working with Network Rail on the necessary infrastructure to extend to Westbury.
Last month, North Somerset Council allocated £15m to the Portishead-line section of the project and approved £11.7m over two years to develop a full business case.
But it still faces a £31m shortfall.
The MetroWest railway expansion plans also include building new railway stations in Henbury, North Filton and Ashley Down.
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