This tunnel is notable for its turreted and castellated north portal with a single-storey cottage on the top, as well as for being the site of a serious accident in 1861 which was influential in the adoption of a robust signalling system in the UK and elsewhere.

[3] Allegedly, the tunnel's architecture had been influenced by William Campion of nearby Danny House, Hurstpierpoint, who lived at Clayton Manor.

[3] The project was not without its critics, the Brighton Guardian, a local newspaper, was commonly skeptical and publicly doubted that it was even possible to construct such a tunnel, and that the line would need to be diverted to avoid the South Downs entirely.

Even more supportive coverage recognised the engineering challenges presented, but acknowledged the competency of the team involved.

[3] The listed north portal of Clayton Tunnel features a complex and distinctive design, being turreted and castellated.

[3] A further feature of the north portal is its attached single-storey cottage, which is perched directly over the line, which has been described as being one of the more unusual and photogenic railway locations in the UK.

[4][5] The West Sussex tunnel was the site of an early form of "automatic" signal invented by CF Whitworth.