These tickets, ranging from as little as £5.40, are available for journeys on the Oxford to London Marylebone line, and on the Birmingham Moor Street to London Marylebone line starting from £6.40.
The discounted tickets are available from 10am on Mondays until midnight on Wednesdays for travel on the Friday of that week.
Specific routes eligible for the discounted tickets include London Marylebone northbound to stations between Bicester North and Birmingham, Birmingham southbound to stations between Banbury and London Marylebone, and journeys between Oxford, Oxford Parkway, Islip and London Marylebone.
In all cases, the tickets will be available in both directions.
Andy Camp, commercial and customer strategy director at Chiltern Railways, said: “The Friday Flash Sale is the perfect opportunity for customers to experience some of the scenic destinations on the Chiltern route and connect with friends and family.
“Tickets can easily be purchased on the Chiltern Railways app or website and we hope this promotion will encourage more people to travel by rail.”
Discounted tickets can be purchased from 10am on September 9 to midnight on September 11 for travel on September 13, from 10am on September 16 to midnight on September 18 for travel on September 20, and from 10am on September 23 to midnight on September 25 for travel on September 27.