Chiltern Railways prior to October 2015 ran a reliable frequent and respectable train service.
Since the introduction of the Oxford parkway route they have reduced trains to commuter stations leading to significant over crowding, cut trains from commuter stations resulting in less choice for commuters and significant over crowding as people take fewer trains. They also can't work their new timetable with the new Oxford trains - which results in poor punctuality and reliability along the line.
They promised they would make changes in December 2015 to fix overcrowding and punctuality / reliability but the changes they made have had no impact on over crowding, punctuality or reliability. They have also not re introduced the trains they removed.