Network Rail and Chiltern Railways platform extension work commences
Platforms at five stations between London Marylebone and Birmingham Moor Street are to be extended to enable longer trains to use them.
Works are taking place at Beaconsfield, High Wycombe, Princes Risborough, Haddenham and Thame Parkway, and Bicester North stations.
Network Rail and Chiltern Railways want space for an extra 1,000 passengers during the morning peak by April 2019.
Nine-carriage trains will start calling at the stations from summer 2015.
The train companies said the move would mean "more seats for passengers on the increasingly busy route, particularly south of Banbury".
Work to lengthen the platforms will take place until 31 July either overnight or during weekends, but some train services will still be affected. Passengers are advised to check for amended timetables online.
Liam Sumpter, area director for Network Rail, said: "As the number of passengers travelling by train on this route continues to rise, it's important that we work together and invest in the network to meet the increasing demand."
Rob Brighouse, managing director for Chiltern Railways, said: "Our passenger-focused investment strategy is at the heart of our business and the lengthening of platforms allows us to deliver long-term, increased capacity where required."