What did happen was total route modernisation. This was an ambitious plan to bring the lines into the modern era of rail travel. Class 115s were replaced by new Class 165s. Semaphore signals were replaced by standard colour light signals and ATP was fitted on the line and trains. Speed limits were increased to 75 mph (only 75 due to running on London Underground track between Harrow and Amersham), all remaining fast loops at stations were removed and the line between and Aynho Junction was singled. Stations were refurbished and even reconstructed (£10 million spent on stations alone), and signal boxes and the freight depots/sidings were demolished. Regular services to Banbury, and a few specials to Birmingham were introduced and a new maintenance depot was built at Aylesbury. This was a massive undertaking and work began in 1988 and by 1992, the route had been completely modernised, demand for the service had grown considerably and the route had become profitable.