A public inquiry will open later into £250m plans for a new rail service between Oxford and London Marylebone.
Chiltern Railways intend to run trains via a new line connecting the Chiltern line to the East-West line.
The proposed service would mean a new station for Kidlington and upgrades for Islip and Bicester stations.
But fears have been raised, including access to an emergency centre during works, congestion at a park-and-ride site and the impact on greenbelt land.
The project would see the loss of green space in Oxford as part of plans for a new station near the Water Eaton park-and-ride site.
While the county council is worried the park-and-ride will not be able to cope with the increase in users due to the new station.
South Central Ambulance Trust has also raised concerns that work could restrict vehicle access at its emergency call centre in Bicester.
The public inquiry will hear all the concerns during a nine-week hearing.
The rail service would be created by upgrading the existing route between Oxford and Bicester Town.
It will also involve building a short connecting line south east of Bicester, linking the Oxford to Bicester and London Marylebone to Birmingham lines.