Hoevet, Joseph (1. 11. 2010). „PENNSYLVANIA RAILROAD – SIGNAL RULES”(PDF). Railroad Signals Landing Page. Архивирано(PDF) из оригинала 7. 10. 2022. г. Приступљено 30. 10. 2022.CS1 одржавање: Формат датума (веза)
networkrail.co.uk
„Signals explained”. Network Rail. 5. 9. 2018. „We have about 40,000 signals across the whole network, mostly colour light signals and with many benefitting from LED technology. There are also still some traditional mechanical semaphore signals.”CS1 одржавање: Формат датума (веза)
