The Joint line ran northwest via Gerrards Cross to High Wycombe, where it met the Wycombe Railway's single-track branch from Maidenhead to Aylesbury via Princes Risborough. North of High Wycombe this line was followed, with significant upgrading of the formation, including dualling and a new alignment for the up line north of Saunderton. The original route, now the down line, has a gradient of 1 in 88, which was too steep for the heavy coal trains run by the GCR. As a result, the two tracks are horizontally and vertically separated, with the down line crossing the Chiltern Hills at the Risborough gap and the up line passing through an tunnel on a flatter gradient.