Owing to the difficult terrain and relatively sparse population between Croydon and Brighton, the line by-passed several towns and villages on the London-Brighton road, such as Reigate and Crawley. Even so, it required substantial earthworks, notably through the North Downs at Merstham, with one of the largest cuttings in Britain; seven tunnels (Merstham, Balcombe, Haywards Heath, Clayton and Patcham initially, then Quarry and Redhill which were constructed later); and several embankments. To avoid steep gradients or detours, the , maximum Ouse Valley Viaduct was built near Balcombe.