They were proposed by the railway engineer J. P. Knight of Nottingham who had adapted this idea from his design of railway signalling systems and constructed by the railway signal engineers of Saxby & Farmer.
De blev foreslået af jernbaneingeniør J. P. Knight fra Nottingham, der havde taget ideen fra sit design af jernbandesignaler og konstrueret af jernbanesignal-ingeniørerne Saxby & Farmer.