Emergency services were called to the scene at 9.05am today (Thursday) following the incident.
Medics from South Central Ambulance Service attended, but the woman was pronounced dead at the scene.
A British Transport Police spokesman said the incident is not being treated as suspicious.
The spokesman said: “Enquiries are ongoing to establish the woman’s identity and inform her family.
“A file will be prepared for the coroner.”
The incident meant that all railway lines running through the area were closed.
Chiltern Railways arranged replacement coaches to run between Bicester North and Princes Risborough.
The line reopened at around 10.30am.
A Chiltern Railways spokesman said: “However, customers should be aware that delays and cancellations will still take place.”