An east-to-south chord between the Great Central Main Line and the Oxford to Bletchley line and was opened on 7 July 1940. Situated about west of Claydon station and north of Calvert railway station at grid reference , it was initially double-track but later singled in 1985. The chord was intended to give added flexibility if bombing disrupted services elsewhere and allowed through working between Bletchley and Aylesbury without the need to reverse at . In the event, the spur never supported a regular passenger service but became a useful route for freight and parcels workings, especially services to Calvert brickworks and trains on diversion such as nighttime sleepers which were sent to during the electrification of the West Coast Main Line, as well as shoppers' specials from Aylesbury to Milton Keynes and movements of empty DMU stock between the Chiltern Main Line and Bletchley maintenance depot. On 2 May 1964, the Midland Pullman worked an 1964 FA Cup Final special via the spur and, for a time, fish trains were reversed down the Great Central here and routed via Bicester London Road and Oxford.